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THE  PRINCIPLES   OF  MYODYNAMICS. 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 
Columbia  University  Libraries 


http://www.archive.org/details/principlesofmyodOOwigh 


THE  PRINCIPLES 


OF  MYODYNAMICS. 


Bv  J.  S.  WIGHT,  M.  D., 

Professor  of  Surgery,   and  Lecturer  on  Physical  Science 
at  the  Long  Island  College  Hospital. 


NEW  YORK: 
BERMINGHAM  &  CO. 


\ 


Copyright,  1881,  by  J.  S.  Wight,  M.  D. 


Printed  by  Samuel  L.  Lyons, 
109  Mercer  St.,  New  York, 


TO 
THE     ALUMNI 

OF  THE 

LONG    ISLAND    COLLEGE    HOSPITAL 

THIS  WORK   IS  INSCRIBED  BY 

THE    AUTHOR. 


PREFACE. 


In  the  following  pages  I  have  made  an 
attempt  to  analyze  the  Principles  of  IMyody- 
namics, — and  hope  those  who  have  not  studied 
the  subject  may  take  the  same  interest  in  it  that 
those  young  men  have  who  have  attended  my 
lectures. 

And  I  may  be  permitted  to  say  that  the 
better  the  surgeon  understands  the  principles  of 
myodynamics,  the  better  he  can  treat  Fractures, 
Dislocations  and  Deformities. 

11^  Pacific  Street,  Brooklyn, 


THE  PRINCIPLES  OF  MYODYNAMICS. 


I.  Myodynamics  treats  of  the  forces  oj  nmscles 
and  their  effects. 

There  are  two  kinds  of  Myodynamics. 

1.  Myostatics,  which  treats  of  muscular  forces, 
when  they  are  in  equiHbrium  with  some  other 
force,  or  forces, — acting  on  a  bon)'  lever. 

II.  Myokinetics,  which  treats  of  muscular 
forces,  when  they  are  moving  some  other  force, 
or  forces, — acting  on  a  bony  lever. 

Examples : — (i.)  When  the  hand  simply  Jiolds 
a  weight, — it  is  a  case  of  myo-statics  :  (2.) 
When  the  hand  moves  a  weight, — it  is  a  case  of 
myo-kinetics. 

2.  In  myo-dynamics  the  principles  of  the 
Lever,  the  Parallelogram  of  Forces,  the  inclined 


lO  THE    PRINXIPLES    OF    MVODVXAMICS. 

Plane,  and  the  Wheel  and  Axle,  are  used.  And 
these  principles  must  be  well  understood. 

3.  There  are  two  principles  involved  in  the 
Lever  : — 

A.     (I.)     In   myostatics  —  The   sum    of    the 

FORCES  ACTING  AT  THE  ENDS  OF  THE  LEVER  EQUALS 
THE    FORCE    ACTLXG    L\    THE    CONTINUITY    OF    THE 

LEVER.     That  is  : — 

I.  Order:  P-fW^-F :  .         .         (i.) 

II.  Order:  P+F=W :  .         .         (2.) 

III.  Order:  F4-\V=:P :  .         .         (3.) 
(II.)    In  myostatics  —  The  Units   of    power 

MULTIPLIED  BY  THE  UNITS  OF  DIS'lANCE  IN  THE 
POWDER-ARM  OF  THE  LEVER  EQUAL  THE  UNITS  OF 
WEIGHT  MULTIPLIED  BY  THE  UNITS  OF  DISTANCE 
IN    THE    WEIGHT- ARM  OF  THE  LEVER.       That    is: 

Px(P-A)=\Vx(W-A). 
In  this  formula   P-A=  the  power-arm,  and 
W-A^  the  weight-arm  of  the  lever. 

(W-A) 

Hence,  P=\V  x :   .         .         .         (i.) 

(p-a) 

(P-A) 

AndW=Px :        .         .         .         (2.) 

(w-a) 


THE    PRINCIPLES    OF    MYODYNAMICS.  I  1 


12  THE    PRINCIPLES    OF    MYODYXAMICS. 

B.  If  the  power  exactly  equals  the  weight,  it 
can  not  move  the  weight  ;  but  if  the  power  is 
greater  than  the  weight,  the  weight  moves,  be- 
cause it  can  not  entirely  resist  the  power  : 
Hence,  in  myokinetics  the  force  of  the  muscles 
is  greater  than  in  myostatics. 

4.  For  the  sake  of  convenience  we  have 
three  orders  of  the  Lever  : 

(i.)  In  the  yfri"/  order  the  fulcrum  is  in  the 
contiiniity  of  the  lever. 

(2.)  In  the  second  order  the  lueight  is  in  the 
contiiuLity  of  the  lever. 

(3.)  In  the  third  order  the  paiuer  is  in  the 
conti7iicity  of  the  lever. 

5.  The  principles  of  the  lever  are  illustrated 
in  Fig.  I. 

(i.)  The  circles  representing  the  forces  at  the 
ends  of  the  lever  are  seen  toofether  in  the  con- 
tinuity  of  the  lever. 

(2.)  The  three  orders  of  the  lever  may  be  re- 
membered by  the  relations  of  the  letters  F.  W. 
P.  as  they  are  located  in  the  continuity  of  the 
lever  :     F.  in  I;    W.  in  II;  P.  in  III  order. 

(3.)  The  equilibrium  of  the  forces  at  the  ends 


THE    PRINCIPLES    OF    MYODYXAMICS.  1 3 

of  the  lever  is  represented  by  the  lever's  hori- 
zontal position. 

6.  There  are  two  principles  in  the  parallelo- 
gram of  forces  : — 

I.  In  myostatics  two  adjacent  sides  of  a  paral- 
lelogram may  represent  two  components,  and 
the  inclosed  diagonal  of  the  parallelogram  may 
represent  the  resultant  of  these  two  components. 
A  force  may  be  resolved  into  two  components 
— as  m  into  a  and  b  :   See  Fig.  2. 

II.  In  myokinetics. — The  forces  are  propor- 
tional to  the  squares  of  the  respective  velocities 
with  which  they  move,  because — 

F 

Kinetic  energy= xV^       .  .  (i.) 

2  g 

W 
Kinetic  energy= xV^       .         .  (2.) 

2  g 

P 
Kinetic  energy:= xV^       .         .         (3.) 

2  g 

Now  g  represents  the  increment  of  velocity 

of  a  falling  body  due  to  gravity,  and  is  32.  16  ft, 

or  9.  8  m. 


14  THE    PRINCIPLES    OF    MVODYNAMICS. 

And  since  the  expression  2  g  is  a  constant,  it 
may  be  rejected  in  comparing  the  energy  of  the 
moving  forces : 

The  energy  of  F:  :FxV^  .         .         (4.) 

The  energy  of  W:  :WxV^       .         .         (5.) 
The  energy  of  P:  :PxV^  .         .         (6.) 

The  fulcrum  is  at  rest  and  has  no  relative 
motion  :  The  power  and  weight  describe  circles, 
whose  radii  are  the  power-arm  and  the  weight- 
arm  of  the  lever  :  Hence,  the  velocity  of  the 
power  is  proportional  to  the  length  of  the  power- 
arm,  and  the  velocity  of  the  weight  is  propor- 
tional to  the  length  of  the  weight-arm  of  the 
lever. 

Example  : — Let  the  power-arm  of  the  radial 
lever  be  2  inches,  the  biceps  brachii  being  the 
power  :  and  let  the  weight-arm  of  the  radial 
lever  be  10  inches,  the  weight  being  5  pounds  : 
Then  the  power  will  be  25  pounds.  Now  sup- 
pose the  power,  in  so  far  as  it  can  balance  the 
weight,  be  a  tangible  body :  Then  we  shall 
have  : 

The  energy  of  P:  :  2  5x2^r:=ioo        .         (7.) 
The  energy  of  W:  : 5x1 0^=000       .         (8.) 


THE    PRINCIPLES    OF    MYODYXAMICS.  10 


1 6  THE    PRINCIPLES    OF    MYODYXAMICS. 

While  the  weight  appears  to  manifest  more 
energy  than  the  power,  it  must  be  kept  in  mind 
that,  in  so  far  as  myostatics  is  concerned,  the 
power  must  be  greater  than  the  weight,  in  order 
to  be  able  to  move  the  weight  :  and  as  the 
weight  moves,  it  accumulates  energy  from  the 
power.  Hence  the  weight,  in  the  case  supposed, 
manifests  the  energy  it  has  obtained  from  the 
power  of  the  contracting  muscle.  Hence,  it 
must  follow,  that  the  energy  gained  by  the  weight 
under  the  circumstances  must  equal  the  energy 
expended  by  the  contracting  muscles — and  that 

IN  EFFECT  THE  WORK    DONE   BY  THE  WEIGHT  MUST 
EQUAL  THE  WORK   DONE  BY  THE  POWER. 

7.  The  principle  of  the  parallelogram  of  forces 
may  be  illustrated  by  Fig.  2. 

(i.)  As  we  shall  resolve  the  forces  of  muscles 
into  reciangitlar  components  Fig.  2,  is  made  a 
rectangle. 

(2.)  In  myostatics  the  force  of  the  muscle  771 
is  resolved  into  the  rectangular  components  a 
and  b, 

(3.)  In  myoki7ietics  the  force  of  the  contract- 
ing muscle  may  be  resolved  into  the  rectangular 


THE    PRI^XIPLES    OF    MYODYXAMICS.  1 7 

components  a  and  b.  The  velocity  of  the  motion 
must  also  be  considered,  according  to  the  prin- 
ciples above  stated. 

8.  The  inclined  plane  involves  the  principle 
of  the  rectanele  of  forces.  For  instance,  when 
two  bones  meet  at  their  articulation  at  an  acute 
or  an  obtuse  angle,  one  rests  upon  the  other  as 
a  body  on  an  inclined  plane. 

9.  Let  F  c  i  Fig.  j,  be  a  movable  bone,  and  o  c 
a  fixed  bone,  c  being  the  joint.  TJie  muscle  m  has 
its  origin  at  o,  and  its  insertion  at  i.  The  force 
of  the  co7itracting  mitscle  is  resolved  into  the  tzuo 
components  a  and  b  :  a  acts  in  the  direction  i  c  and 
is  a  displacing  component  acting  on  the  movable 
bone.  But  b  acts  in  the  direction  i  P  and  c ordains 
a  DISPLACING  and  a  retentive  component.  A^oiu 
the  force  b  acts  at  i  and  reacts  at  o  in  the  direction 
0  c  :  The  fixed  bone  rests  on  the  mailable  bone  as 
a  body  rests  on  an  inclined  plane  :  TJie  fixed 
bone  tends  to  descend  on  the  movable  bone — which  is 
impossible :  Hence  the  movable  bone  tends  to  go 
upward  binder  tJie  fixed  bone  with  the  same  force 
that  the  fixed  bone  tends  to  descend:  And  this 
force  luJiicJi  is  equal  b  is  resolved  into  the  compon- 


i8 


THE    PRLXCIPLES    OF    MYODYNAMICS. 


THE    PRINCIPLES    OF    MYODYXAMICS.  1 9 

ents  C R,or  do  and cd :  The  component  d o presses 
the  movable  bone  against  the  fixed  bone.  The  com- 
ponent d  0  is  a  MOVING  or  retentive  component. 
And  the  component  c  d  is  a  displacing  component 
acting  ifi  the  direction  c  d.  Bnt  zae  have  already 
a  displacing  component  i  c  acting  in  the  opposite 
direction,  luhich  zuill  leave  a  displacing  component 
equal  i  d.  That  is  we  must  dedjict  from  the  com- 
ponent c  i  d  the  component  i  c  luhich  zuill  leave  the 
component  i  d.  Thatzvill  be  the  displacing  coi?ipo7i- 
ent  of  the  cojitracting  muscle  m.  The  lines  o  d  and 
i  d  represent  the  rectangular  components  of  the 
muscle.  In  a  similar  mangier  all  rectangular 
components  77iay  be  found. 

10.  The  wheel  and  axle  act  on  the  principle 
of  the  lever. 

Example :  The  rotation  of  the  skull  on  the  top 
of  the  spinal  column  is  an  example  of  this  prin- 
ciple. 

1 1 .  The  agents  which  move  the  human  body 
are  bones  and  muscles.  The  bones  are  passive 
agents,  and  the  muscles  are  active  agents. 

12.  Bones  in  regard  to  the  action  of  muscles 
may  be  divided  \x\\.o  fixed  and  movable  : — 


20  THE    PRINXIPLES    OF    MVODYXAMICS. 

(i.)  A  fixed  bone  gives  origin  to  the  moving 
muscle — but  does  not  move. 

(2.)  A  movable  bone  gives  insertion  to  the 
movino-  muscle — -and  is  moved. 

(3.)  In  some  cases  the  movable  bone  becomes 
fixed,  then  the  fixed  bone  may  move. 

(4.)  When  a  muscle  spans  two  joints  and  the 
interjacent  bones,  there  may  be  one  or  two 
movable  bones  : — 

(i.)  One  of  the  adjacent  bones  may  move, 
when  the  other  adjacent  bone  and  the  interjacent 
bone  will  be  fixed. 

(ii.)  The  two  adjacent  bones  may  move,  when 
the  interjacent  bone  will  be  the  fixed  bone  : — 

(iii.)  One  of  the  adjacent  bones  and  the  inter- 
jacent bone  may  move,  when  the  other  adjacent 
bone  will  be  the  fixed  bone. 

13.  A  movable  bone  may  be  a  lever  of  the 
first,  second,  or  third  order  : 

Examples,  (i.)  The  skull  acted  on  by  the 
posterior  cervical  muscles  is  a  lever  of  the  first 
order. 

(2.)  The  foot  acted  on  by  the  muscles  of  the 
tendo-Achillis  is  a  lever  of  the  second  order. 


THE    PRINCIPLES    OF    MYODYNAMICS.  2  1 


2  2  THE    PRINCIPLES    OF    MYODYNAMICS. 

(3.)  The  radius  acted  on  by  the  biceps  brachii 
is  a  lever  of  the  third  order. 

14.  According  to  the  direction  of  their  fibres, 
muscles  are  of  three  kinds  :  direct,  oblique,  and 
radiate. 

(i.)  In  a  direct  fibred  muscle  the  fibres  run  in  a 
longitudinal  direction  from  the  tendon  of  origin  to 
the  tendon  of  insertion,  as  is  shown  in  I.  Fig.  4. 

(2.)  In  an  obliqite fibred  muscle  the  fibres  run 
in  an  oblique  direction  from  the  tendon  of  origin 
to  the  tendon  of  insertion,  as  is  shown  in  II,  III, 
IV,  Fig.  4. 

There  are  three  kinds  of  oblique  fibred  mus- 
cles :  The  sJwrt  obliqite  (II),  the  long  oblique 
(III),  and  the  double  oblique  (IV). 

(3.)  In  the  radiate  fibred  muscle  the  fibres  run 
in  radiate  directions  from  a  central  tendon  to 
various  points  of  insertion  (V). 

Examples  : — 

(i.)  The  sartorius  is  a  direct  fibred  muscle. 

(ii.)  The  extensor  proprius  pollicis  is  a  short 
oblique  fibred  muscle. 

(iii.)  The  semimembranosus  is  a  long  oblique 
fibred  muscle. 


THE    PRINXIPLES    OF    MYODYNAMICS.  23 

(iv.)  The  rectus  femoris  is  a  double  oblique 
fibred  muscle. 

(v.)  The  diaphragm  is  a  radiate  fibred  muscle. 

1 5.  In  regard  to  m.otion  and  power  as  ex- 
hibited by  muscles  the  following  statements  may 
be  made  : — 

( I .)  The  longer  the  fibres  of  a  muscle  the  more 
extensive  is  the  motion  it  can  make  and  the 
greater  its  power. 

(2.)  The  power  of  a  muscle  increases  as  the 
number  of  its  fibres  increases  :  The  muscle 
whose  transverse  section  has  the  greatest  area 
will  have  the  greatest  power. 

(3.)  Other  things  being  equal,  the  proper  use 
of  a  muscle  will  increase  its  power  to  a  certain 
extent. 

(4.)  The  volition  can  be  made  to  act  on  part 
of  the  fibres  of  some  muscles,  as,  for  instance, 
the  gluteus  medius,  or  the  pectoralis  major. 

(5.)  The  energy  of  a  contracting  muscle  is 
proportional  to  the  energy  of  the  volition. 

(6.)  The  firmness  of  a  contracting  muscle  is 
proportional  to  the  energy  with  which  it  con- 
tracts. 


24  THE    PRINCIPLES    OF    MYODYNAMICS. 

1 6.  In  a  human  body,  whose  entire  weight  is 
1432  pounds,  the  recent  bones  weigh  2 1 J  pounds, 
and  the  recent  muscles  weigh  ']^\  pounds  :  and 
the  weight  of  the  bones  is  to  the  weight  of  the 

muscles  as, 

43  :  i55  ; 

and  the  weight  of  the  muscles  is  to  the  weight 

of  the  body  as, 

155:287. 

It  is  easy  to  determine  from  the  weights  and 
the  specific  gravities  of  bones  and  muscles,  that, 
in  the  hitman  body,  the  bnlk  of  the  muscles  is  more 
tha7i  six  times  the  bulk  of  the  bones, 

17.  The  force  of  a  contracting  muscle 
acting  on  a  bony  lever  of  the  first,  second, 
or  third  order  is  resolved  into  two  rec- 
tangular components. 

(a)  One  of  these  components  tends  to  move 
the  movable  bone,  and  may  be  called  the  movi^ig 
componejit :  (b)  The  same  component  tends  to 
hold  the  movable  bone  and  the  fixed  bone  to- 
gether, and  may  be  called  the  rete7ttive  com- 
ponent, (c)  The  other  of  these  components  tends 
to  displace  the  movable  bone  from   the    fixed 


THE    PRINXIPLES    OF    MYODYNAMICS.  2D 


2  6  THE    PRINCIPLES    OF    MYODYNAMICS. 

bone,  and  may  be  called  the  displacing  com- 
ponent : —  The  conformation  of  the  joint  may  be 
such  that  the  moving  component  is  a  displacing 
component :  The  movifig  component  of  the  biceps 
brachii  is  a  displacing  component. 

1 8.  The  long  axis  of  the  contracting  muscle, 
the  distance  in  the  lono-  axis  of  the  movable  bone 
from  the  insertion  of  the  contractino-  muscle  to 
the  centre  of  the  joint,  and  the  distance  in  the 
long  axis  of  the  fixed  bone  from  the  centre  of 
the  joint  to  the  origin  of  the  contracting  muscle 
make  the  sides  of  a  triangle,  that  may  be  called 
the  myodynamic  triangle. — In  Fig.  5,  o  i  c  is  the 
myodynamic  triangle. 

19.  That  angle  of  the  myodynamic  triangle 
included  between  the  lono-  axis  of  the  contract- 
ing  muscle  and  the  long  axis  of  the  movable 
bone  is  important,  and  ma)'  be  called  the  myo- 
dynamic angle.  In  Fig.  5,  o  i  c  is  the  myody- 
namic angle. 

(a)  In  the  case  of  a  bony  lever  of  any  order,  the 
myodyiiamic  angle  is  the  guide  to  the  mag7iitudes 
of  the  rectangidar  coj?ipone7its  of  the  contracting 
muscle. 


THE    PRINCIPLES    OF    MYODYNAMICS.  2  7 


2S  THE    PRINCIPLES    OF    MYODYNAMICS. 

(b)  The  myodynamic  angle  must  be  acute, 
rigJit,  or  obtitse,  according  to  the  extent  of  the 
contraction  of  the  moving  muscle. 

20.  The  ulna  may  be  an  example  of  a  bony 
lever  of  the  first  order  :  The  humerus  will 
be  the  fixed  bone,  and  the  triceps  brachii  will 
be  the  contracting  muscle  :  The  myodynamic 
angle  will  be  between  the  triceps  brachii  and  the 
ulna. 

21.  In  a  bony  lever  of  xh^  first  order,  let  a  c 
i  be  a  movable  bone,  oca  fixed  bone,  c  their 
joint,  m  the  contracting  muscle,  whose  origin 
and  insertion  are  at  o  and  i  :     See  Figs.  5,  6,  7. 

The  power  is  at  i,  the  fulcrum  is  at  c,  and  the 
weight  is  at  a.  The  myodynamic  triangle  is  o  i 
c.     And  the  myodynamic  angle  is  o  i  c. 

(a)  The  myodynamic  angle  is  acitte  in  Fig.  5, 
right  in  Fig.  6,  and  obtitsc  in  Fig.  7. 

(b)  From  the  origin  of  the  contracting  muscle 
draw  a  perpendicular  to  the  long  axis  of  the 
movable  bone — prolonged  if  necessary,  complete 
the  rectangle  o  b  i  d,  whose  diagonal  is  the  long 
axis  of  the  contractino^  muscle  :  Also  construct 
the    rectanorle  d  o  R  c,  whose  diao^onal  is  the 


THE    PRINCIPLES    OF    MVODVNAMICS.  29 


30  THE    PRIXCIP1.es    OF    MYODYXAMICS. 

lonor  axis  of  the  fixed  bone  fi'om  the  oriein  of 
the  contracting  muscle  to  the  joint. 

(c)  When  the  myodynamic  angle  is  right,  the 
longr  axis  of  the  contractinor  muscle  will  be  the 
perpendicular  drawn  from  the  origin  of  the  mus- 
cle to  the  movable  bone  :  The  component  o  d 
will  coincide  with  the  long  axis  of  the  muscle  ; 
and  there  will  be  one  rectangular  component. 
See  Fig.  6.  The  component  d  c  which  is  sup- 
posed to  make  part  of  the  resultant  m  is  exactly 
equal  the  component  c  d,  which  makes  part  of 
the  resultant  s  i,  or  o  c  :  and  these  components 
neutralize  each  other. 

(d)  When  the  myodynamic  angle  is  acute,  or 
obtuse,  the  perpendicular  drawn  from  the  origin 
of  the  contractinor  muscle  to  the  movable  bone 
will  fall  outside  the  lonof  axis  of  the  muscle,  and 
there  will  be  two  rectangular  components.  See 
Figs.  5  and  7.  The  displacing  component  will 
be  represented  by  the  line  i  d. 

22.  A.  When  the  myodynamic  angle  varies, 
the  magnitudes  of  the  rectangular  components 
will  vary.      The  following  laws  of  variation  in 


THE    PRINXIPLES    OF    MVODVNAMICS.  3 1 

the  magnitudes  of  the  rectangular  components 
may  be  enunciated. 

(i.)  The  displacing  component  (i  d)  of  the 
contracting  muscle  will  be  a  maximum,  when 
the  myodynamic  angle  is  least,  and  will  con- 
stantly diminish  till  the  myodynamic  angle  is 
7Hght, — when  there  will  be  no  displacing  com- 
ponent. 

(2.)  The  displacing  component  (i  d)  of  the 
contracting  muscle  will  constantly  increase  from 
the  zero-point — where  the  myodynamic  angle  is 
right — -till  the  myodynamic  angle  is  greatest. 

(3.)  The  moving — or  retentive — component 
(o  d)  of  the  contracting  muscle  will  be  a  mini- 
mum, when  the  myodynamic  angle  is  least,  and 
will  constantly  increase  till  the  myodynamic 
anofle  is  ri^ht,  when  it  will  be  a  maximum. 

(4.)  The  moving,  or  retentive  component  (o  d) 
of  the  contracting  muscle  will  constantly  dimin- 
ish from  the  maximum  point — where  the  myody- 
namic angle  is  right — till  the  myodynamic  angle 
is  greatest. 

22.  B.  In  Figs.  5,  6,  and  7,  on  the  long 
axis    of  the    contractino-    muscle    construct    the 


32  THE    PRINCIPLES    OF    MYODYNAMICS. 


THE    PRINCIPLES    OF    M VODVN AMICS.  33 

paralleloi^ram  o  s  i  c,  which  shall  correspond 
to  and  be  twice  as  great  as  the  myodynamic 
triangle  : — 

(i.)  In  F'ig.  5,  the  components  i  c  and  c  d  co- 
operate in  the  direction  i  c  d,  making  the  entire 
displacing  component  equal  i  d  :  The  whole 
force  of  the  contracting  muscle  appears  in  the 
rectangular  components. 

(2.)  In  Fig.  6,  the  component  d  c  and  the  com- 
ponent c  d  are  antagonistic  and  equal,  and  there- 
fore neutralize  each  other,  so  that  there  is  no 
displacing  component  :  The  whole  force  of  the 
contracting  muscle  appears  in  the  moving  com- 
ponent. 

(3.)  In  Fig.  7,  the  component  c  d  is  greater 
than  the  component  i  c  by  the  component  i  d. 
which  is  the  displacing  component  :  The  whole 
force  of  the  contracting  muscle  appears  in  the 
rectangular  components. 

23.  In  a  bony  lever  of  the  second  order,  let  a 

c  i  be  a  movable  bone,  oca  fixed  bone,  c,  their 

joint,  m,  the  contracting  muscle,  whose  origin 

and  insertion  are  at  o  and  i.     See  F'igs.  8,  9, 

lO.     The  power  is  at  i,  the  weight  at  c,  and  the 


34 


THE    PRINCIPLES    OF    MYODYNAMICS. 


THE    PRINCIPLES    OF    MYODYNAMICS.  35 

fulcrum  at  a.     The  myodynamic  triangle  is  o  i 
c.     And  the  myodynamic  angle  is  o  i  c. 

(a)  The  myodynamic  angle  is  acute  in  Fig.  8, 
right  in  Fig.  9,  and  obtuse  in  Fig.  10. 

(b)  From  the  origin  of  the  contracting  muscle 
draw  a  perpendicular  to  the  long  axis  of  the 
movable  bone — prolonged  if  necessary.  Com- 
plete the  rectangle  o  b  i  d,  whose  diagonal  is  the 
long  axis  of  the  contracting  muscle. 

(c)  As  before,  when  the  myodynamic  angle  is 
7'ight,  there  will  be  one  rectangular  component. 

(d)  Also  as  before,  when  the  myodynamic 
angle  is  acute  or  obtiise,  there  will  be  two  rec- 
tangular components. 

24.  A.  When  the  myodynamic  angle  varies, 
the  magnitudes  of  the  rectangular  components 
will  vary.  The  laws  of  variation  in  the  magni- 
tudes of  the  rectangular  components  will  be  the 
same  in  a  bony  lever  of  the  second  order  as  in 
a  lever  of  the  first  order  :  See  22,  (i),  (2),  (3), 

(4). 

24.  B.  In  Figs.  8,  9  and  10,  on  the  long  axis 
of  the  contracting  muscle  construct  the  parallelo- 
gram o  s  i  c,  which  shall  correspond  to  and  be 


THE    PRINCIPLES    OF    MVODVXAMICS. 


cf^jo^ 


.'\^?JV 


17.  Orcfer. 


THE    PRINCIPLES    OF    MYODYNAMICS.  37 

twice  as  great  as  the  myodynamic  triangle  : — 
Then  the  same  conclusions  may  be  drawn  as  in 

(I,)(2),   (3),    22    B. 

2  5.  In  a  bony  lever  of  the  tJiird  order,  let  a  c 
i,  be  a  movable  bone,  oca  fixed  bone,  c,  their 
joint,  m,  the  contracting  muscle,  whose  origin 
and  insertion  are  at  o  and  i  :  See  Figs.  1 1,  12, 
I  3.  The  power  is  at  i.  the  weight  at  a,  and  the 
fulcrum  at  c.  The  myodynamic  triangle  is  o  i  c. 
And  the  myodynamic  angle  is  o  i  c. 

(a)  The  myodynamic  angle  is  acute  in  Fig.  i  i , 
right  in  Fig.  12,  and  obtuse  in  Fig.  14. 

(b)  From  the  origin  of  the  contracting  muscle 
draw  a  perpendicular  to  the  long  axis  of  the 
movable  bone — prolonged  if  necessary.  Com- 
plete the  rectangle  o  b  i  d,  whose  diagonal  is 
the  long  axis  of  the  contracting  muscle. 

(c)  Again  as  before,  when  the  myodynamic 
angle  is  right,  there  will  be  one  rectangular  com- 
ponent. 

(d)  Also  as  before,  when  the  myodynamic 
angle  is  acute  or  obtuse,  there  will  be  two  rect- 
angular components. 

26  A.  In  this  case  also,  when  the  myodynamic 


38 


THE    PRINCIPLES    OF    MYODYNAMICS. 


THE    PRINXIPLES    OF    MYODYNAMICS.  39 

angle  varies,  the  magnitudes  of  the  rectangular 
components  will  vary  : — and  the  laws  of  varia- 
tion in  the  magnitudes  of  the  rectangular  com- 
ponents will  be  the  same  as  those  of  a  bony  lever 
of  the  first,  or  second  order  : — See  22,  (i),  (2), 

(3).  (4). 

26.   B.  In   Figs.    II,  12    and   13,  on  the  long 

axis   of  the    contracting    muscle    construct    the 

parallelogram  o  s  i  c  which  shall  correspond  to 

and  be  twice  as  great  as  the  myodynamic  tri- 

anHe  : — 

o 

(i.)  In  Fig.  II,  the  whole  force  of  the  con- 
tracting muscle  appears  in  the  rectangular  com- 
ponents. 

(2.)  In  Fig.  12,  the  whole  force  of  the  con- 
tracting muscle  appears  in  the  moving  compo- 
nent. 

(3.)  In  Fig.  13,  the  whole  force  of  the  con- 
tracting muscle  appears  in  the  rectangular  com- 
ponents. 

26.  C.  Rules  for  finding  the  rectangular  com- 
ponents of  a  contracting  muscle  : — 

(a)  Draw  a  line  from  the  origin  of  the  contract- 
ing iniLScle  at  right  angles  to  and  nieeting  the  long 


40  THE    PRINCIPLES    OF    MVODVN AMICS. 


THE    PRINCIPLES    OF    MVODVNAMICS.  4 1 

axis  of  the  iiiovable  bone,  prolonged  if  necessary, 
and  it  will  represent  the  moving,  or  retentive  com- 
ponent. 

(b)  Drazu  a  line  in  the  long  axis  of  the  movable 
bone  from  the  insertion  of  the  contracting  muscle 
till  it  meets  the  line  of  the  moving  component  at 
right  angles,  and  it  zuill  represe7it  the  displacing 
component. 

27.  Let  i  d  and  o  d  be  the  lines  representing 
the  rectangular  components  of  the  contracting 
muscle,  whose  long  axis  is  o  i  :  See  Fig.  14. 
Let  a  and  b  be  the  rectangular  components,  and 
m  the  force  of  the  contractinor  muscle. 

(a)  Then  it  is  evident  that  we  shall  have  the 
following  proportions  : 

a:b::di:do  ..         .  .  (i); 

a  :  m  :  :  d  i  :  i  o  .  .  .  (2) ; 

b  :  m  :  :  d  o  :  i  o  .  .  .  (3) ; 

(b)  From  these  proportions  we  may  derive 
the  following  equations  :— - 

di 

a=b  X .  .  .      •    .  (i) ; 

d  o 


42  THE    PRINCIPLES    OF    MVODYXAMICS. 


THE    PRINCIPLES    OF    MYODVXAMICS.  43 

di 

a=m  X .         .  .         .  (2) ; 

i  o 

do 

b=:a  X ...         .  (3) ; 

di 

do 

br=m  X .  .  .  .  (4)  ; 

i  o 

i  o 

m=a  X .         .         .  .  (5) ; 

di 

i  o 

m:=:b  X .  .  .  .  (6) ; 

do 

28.  It  will  be  convenient  to  call  the  right 
angled  triangle  o  d  i,  Fig.  14.  the  coinponcnt  tri- 
angle. Hence,  by  the  above  formulae,  when  two 
sides  of  the  component  triangle  and  one  of  the 
related  forces  are  known,  it  is  possible  to  find 
either  of  the  other  two  forces.  It  will  not  be 
necessary  to  translate  these  formulae  into  written 
rules. 

29.  A.  In  so  far  as  the  force  of  the  contract- 
ing muscle  is  concerned,  the  joint  between  a 
movable  bone  and  a  fixed  bone  will  be  in  a  con- 


44  I'HE    PRINXIPLES    OF    MVODYNAMICS. 


THE    PRINXIPLES    OF    MYODYXAMICS.  45 

dition  of  maximum  stability,  when  the  myody- 
namic  angle  is  a  right  angle,  in  the  case  of  any 
bony  lever.  But  the  stability  of  the  joint  be- 
comes relatively  less  as  the  myodynamic  angle 
differs  from  a  right  angle,  in  the  case  of  any 
bony  lever.  The  displacing  component  is  a 
factor  of  instability,  and  is  antagonized  by  the 
capsular  ligaments. 

29.  B.  In  the  case  of  any  order  of  bony 
lever,  the  following  general  conclusions  may  be 
drawn  : — 

(i.)  When  the  myodynamic  angle  is  acute  a 
greater  part  of  the  force  of  the  contracting  muscle 
will  be  a  displacing  component  than  when  this 
angle  is  obtuse. 

(2.)  When  the  myodynamic  angle  is  obtuse  a 
greater  part  of  the  force  of  the  contracting 
muscle  will  be  a  retentive  component  than  when 
this  anorle  is  acute. 

(3.)  Hence  with  the  same  muscular  force  an 
obtuse  myodynamic  angle  will  afford  conditions 
of  greater  stability  to  the  joint  than  an  acute 
myodynamic  angle. 

(4.)  But  other  things  being  equal,  a  right-myo- 


46 


THE    PRINCIPLES    OF    MYODYNAMICS. 


THE    PRINCIPLES    OF    MYODYNAMICS.  47 

dynamic  angle  will  give  the  most  stable  relations 
to  a  joint. 

30.  In  myodynamics,  when  we  have  any  order 
of  bony  lever,  the  force  of  the  weight  may  be 
resolved  into  rectangular  components. 

(i.)  One  component  of  the  w^eight  acts  at 
right  angles  to  the  long  axis  of  the  movable 
bone  :  as  this  component  resists  the  moving 
component  of  the  contracting  muscle,  it  may  be 
called  the  Resisting  component ;  and  as  it  aids  in 
holding  the  movable  bone  against  the  fixed  bone, 
it  may  also  be  called  the  Retentive  conipone7it. 
See  a  d,  Fior.  i5. 

o 

(2.)  The  other  component  of  the  weight  acts 
in  the  direction  of  the  lone  axis  of  the  movable 
bone  :  as  this  component  tends  to  displace  the 
movable  bone  from  the  fixed  bone,  it  may  be 
called  the  Displacing conipoiient.    See  a  e.  Fig.  1 5. 

31.  The  two  lines  which  represent  the  com- 
ponents of  the  weight  are  the  base  and  perpen- 
dicular of  a  right-angled  triangle — that  may  be 
called  the  dynamic  triangle.  One  angle  of  this 
triangle  is  important,  and  may  be  called  the 
dynamic  angle. 


48  THE    PPaNCIPLES    OF    MYODYNAMICS. 


THE    PRINXIPLES    OF    MYDDVNAMICS.  49 

32.  In  Figs.  1 5,  16  and  17,  let  f  o  c  be  the 
fixed  bone,  a  c  b  the  movable  bone,  and  c  their 
joint  :  in  Figs.  i5  and  16,  the  joint  is  in  the  con- 
tinuity of  the  lever.  Let  P  be  the  power,  F  the 
fulcrum,  and  W  the  weight,  whose  tendency  is 
in  the  direction  a  w.  On  the  line  a  w,  as  a 
diaofonal,  construct  the  rectancrle  a  e  w  d,  one  of 
whose  sides  lies  in  the  long  axis  of  the  movable 
bone — prolonged  if  necessary.  T/ie  dyiianiic  tri- 
angle is  a  e  IV.  The  dynamic  angle  is  included 
behueen  the  long  axis  of  the  movable  bone — p7V- 
longed  if  necessary,  aiid  the  line  of  the  tendency 
of  the  zueight,  and  is  e  a  zu. 

33.  The  dynamic  angle  is  generally  acute.  It 
may  be  a  right-angle  at  one  point  only — but  is 
never  greater  than  a  right-angle. 

34.  In  Figs.  i5,  16  and  17,  illustrating  respec- 
tively levers  of  the  first,  second  and  third  orders, 
the  line  a  d  represents  the  resisting  compon- 
ent of  the  weight,  and  the  line  a  e  represents  the 
displacing  component  of  the  weight. 

35.  In  regard  to  the  magnitudes  of  the  7'ec- 
tang7(lar  compone?its  of  the  weight,  the  following 
conclusions  may  be  drawn  : 


5o  THE    PRINCIPLES    OF    MYODYNAMICS. 

(i.)  The  resisting  component  of  the  weight 
is  greatest,  when  the  dynamic  angle  is  a  right- 
ano^le. 

(2.)  The  resisting  component  of  the  weight 
is  least,  when  the  dynamic  angle  is  the  most 
acute. 

(3.)  When  there  is  no  dynamic  angle,  there  is 
no  resisting  component. 

(4.)  The  displacing  component  of  the  weight 
is  greatest,  when  the  dynamic  angle  is  the  most 
acute. 

(5.)  When  the  dynamic  angle  is  a  right- 
angle,  the  weight  has  no  displacing  component. 

(6.)  In  general  the  resisting  component  aug- 
ments up  to  a  certain  point  and  then  diminishes 
in  magnitude. 

(7.)  In  general  the  displacing  component  di- 
minishes up  to  zero  and  then  augments  in  mag- 
nitude. 

36.  In  so  far  as  the  force  of  the  weight  is  con- 
cerned, i\\^  joint  between  a  movable  bone  and  a 
fixed  bone  is  in  a  condition  of  maximum  stability, 
when  the  dynamic  angle  is  a  right-angle,  in  the 
case  of  any  bony  lever.     But  the  stability  of  the 


THE    PRINCIPLES    OF    MYODYNAMICS.  5 1 


^2  THE    PRINCIPLES    OF    MYODYNAMICS. 

joint  becomes  relatively  less  as  the  dynamic 
angle  becomes  less  than  a  right-angle,  in  the 
case  of  any  bony  lever.  The  displacing  compon- 
ent is  a  factor  of  instability. 

37.  By  the  formulae  of  the  component  tri- 
angle, 27,  (b)^  when  two  sides  of  the  dynamic 
triangle  and  one  of  the  related  forces  are  known, 
it  is  possible  to  find  either  of  the  other  two 
forces. —  The  dynamic  triangle  is  the  component 
triangle  of  the  zueight. 

38.  There  are  two  important  facts  in  regard  to 
the  structure  of  joints  that  ought  to  be  remem- 
bered : — In  myodynamics  (a)  the  ligaments  al- 
ways and  (b)  the  conformation  of  the  joint 
sometimes  antagonize  the  displacing  components 
of  the  power  and  the  weight.  The  crucial  liga- 
ments of  the  knee-joint  and  the  conformation  of 
the  hip-joint  are  especially  adapted  to  antagonize 
displacing  components. — In  the  wrist  joint  the 
displacing  components  are  small  and  the  reten- 
tive components  are  large. 

39.  When  the  displacing  components  are  suc- 
cessfully antagonized  by  the  articular  ligaments 
and    the    articular    conformation,  the    forces  of 


THE    PRINCIPLES    OF    MVODYNAMICS.  63 

these  components   are  brought  to  bear   on   the 
joint-surfaces  of  the  movable  and  fixed  bones. 

40.  The  combined  effects  of  the  rectangular 
components  of  the  power  and  the  weight  on  a 
bony  lever  of  any  order  may  now  be  considered. 

(a)  The  following  formulae  may  be  noted  in 
regard  to  the  three  orders  of  the  lever : 

I.  p+W^F  ....         (i.) 

II.  p_^F=\V         ....  (2.) 

III.  F+\V=P       ....         (3.) 
In  the  above  formulae : 

(i.)  P  is  the  resultant  of  the  rectangular  com- 
ponents of  the  pcnver. 

(ii.)  W  is  the  resultant  of  the  rectangular 
components  of  the  weight. 

(iii.)  F  is  the  residtant  of  the  rectangular  com- 
ponents of  xh&fulcricjn. 

(b)  In  this  place  it  may  be  remarked  that  the 
rectangular  components  of  the  fulcrum  may  be 
analyzed  in  the  same  manner  as  the  rectangular 
components  of  the  power  and  the  weight,  in  the 
case  of  any  order  of  bony  lever. 

(c)  In  regard  to  the  moving  components  of 
the  power  and  the  resisting  components  of  the 


:)4  THE    PRINXIPLES    OF    MVODVXAMICS. 

weight  the  following  conclusions  may  be  drawn  : 

(i.)  In  a  bony  lever  of  the  I  order: — The 
moving  component  of  the  power  plus  the  resist- 
ing component  of  the  weight  equals  the  pressure 
of  these  components  on  the  fixed  bone. 

(2.)  In  a  bony  lever  of  the  II  order: — The 
resisting  component  of  the  weight  niiniLS  the 
moving  component  of  the  power  equals  the 
pressure  of  the  components  on  the  fulcrum. 

(3.)  In  a  bony  lever  of  the  III  order: — The 
moving  component  of  the  power  minus  the  re- 
sisting component  of  the  weight  equals  the 
pressure  of  these  components  on  the  fixed  bone. 

(4.)  The  sum  of  the  components  acting  at 
right  angles  to  the  ends  of  the  bony  lever  equals 
the  resultant  component  acting  in  the  continuity 
of  the  lever. 

(5.)  The  moving  and  resisting  components  are 
conservative. 

(d)  In  regard  to  the  displacing  components  of 
the  power  and  the  weight  the  following  conclu- 
sions may  be  drawn  : 

(i.)  In  a  bony  lever  of  the  I  order: — i.  Begin 
with  an  acute  myodynamic  angle,  when  the  dy- 


THE    PRIN'CIPLES    OF    MYODYNAMICS.  55 

namic  ans^le  is  also  acute,  and  move  the  weig^ht 
till  the  dynamic  angle  is  a  right  angle  : — See 
Fig.  1 5.  During  this  motion  the  displacing 
components  of  the  power  and  weight  will  co- 
operate,— ii.  As  the  bony  lever  moves  from  the 
line  where  the  dynamic  angle  is  a  right  angle  to 
the  line  where  the  myodynamic  angle  is  a  right- 
angle,  the  displacing  components  of  the  power 
and  the  weight  will  antagonize. — iii.  As  the  bony 
lever  moves  from  the  line  where  the  myody- 
namic angle  is  a  right  angle,  the  displacing  com- 
ponents of  the  power  and  weight  will  co-operate. 
— iv.  When  the  myodynamic  triangle  is  right- 
angled  at  the  joint,  the  resisting  component  of 
the  weight  equals  the  displacing  component  of 
the  power. — This  fact  can  be  demonstrated  by 
the  myometer. 

(2.)  In  a  bony  lever  of  the  II  order  : — i. 
During  the  motion  of  the  lever  from  the  most 
acute  myodynamic  angle  to  the  right-angled 
dynamic  angle,  the  displacing  components  of 
the  power  and  weight  will  co-operate. — ii.  During 
the  motion  of  the  lever  from  the  riofht-anorled 

o  o 

dynamic  angle  to  the  right-angled  myodynamic 


56  THE    PRINCIPLES    OF    MYODYXAMICS. 

angle,  the  displacing  components  of  the  power 
and  weight  will  antao-onize. — iii.  After  the  lever 
moves  from  the  line  where  the  myodynamic 
angle  is  right-angled,  the  displacing  components 
of  the  power  and  weight  co-operate. — iv.  When 
the  myodynamic  triangle  is  right-angled  at  the 
joint,  the  resisting  component  of  the  weight 
equals  the  displacing  component  of  the  power. 

(3.)  In  a  bony  lever  of  the  III  order: — i. 
Durine  the  motion  of  the  lever  from  the  most 
acute  myodynamic  angle  to  the  right-angled 
dynamic  angle,  the  displacing  components  of 
the  power  and  weight  antagonize. — ii.  While 
the  lever  is  moving  from  the  right-angled  dy- 
namic angle  to  the  right-angled  myodynamic 
angle,  the  displacing  components  of  the  power 
and  weight  co-operate, — iii.  After  the  lever 
moves  from  the  line  of  the  right-angled  myody- 
namic angle,  the  displacing  components  of  the 
power  and  weight  antagonize. — iv.  Again,  when 
the  myodynamic  triangle  is  right  angled  at  the 
joint,  the  resisting  component  of  the  weight 
equals  the  displacing  component  of  the  power. 
41.  When  the  myodynamic  triangle  is  right- 


THE    PRINCIPLES    OF    MVODYNAMICS.  dj 

ang-led  at  the  joint,  the  weight  may  equal  the 
displacing  component  of  the  contracting  muscle. 
The  sides  of  the  myodynamic  triangle  may  be 
measured,  and  then  it  is  easy  to  find  the  mov- 
ing component  as  well  as  the  entire  force  of  the 
contractinor  muscle. 

Example. — Let  \V:=5o  :  The  displacing  com- 
ponent of  the  contracting  muscle^ 5o.  Let  the 
sides  of  the  component  triangle  be  2,  10,  and 
10.19  respectively:  Tnen  by  the  formulae  (3) 
and  (5)  27  (b.) 

10 

m-c:=5o  X =25o  : 

2 

10.19 

And  f-m=:5ox =254.75. 

2 

In  which  m-c  is  the  moving  component  and 
f-m  the  entire  force  of  the  contracting  muscle. 

(A.)  It  is  evident  that  the  displacing  com- 
ponent of  the  power  is  antagonized  by  the 
articular  lioraments  that  connect  the  movable 
and  the  fixed  bones :  Hence  the  displacing 
component  of  the  power  is  combined  with  the 
moving   component  of   the  power  and    presses 


58  THE    PRINCIPLES    OF    MYODVXAMICS. 

the  movable    bone   ao^alnst    the    fixed    bone  as 
long  as  the  articular  ligaments  remain  unbroken. 

(B.)  It  is  also  evident  that  the  displacing 
component  of  the  weight  is  antagonized  by  the 
articular  ligaments  that  connect  the  movable 
and  the  fixed  bones :  Hence  the  displacing 
component  of  the  weight  is  combined  with  the 
resisting  component  of  the  weight  and  presses 
the  movable  bone  aeainst  the  fixed  bone  as 
lono^  as  the  articular  lio^aments  remain  unbroken. 

(C.)  A  case  might  arise  in  which  the  entire 
force  of  the  muscles  that  span  a  joint  would  act 
as  a  retentive  force  :  for  instance,  when  a  weiofht 
is  lifted  by  the  hand  in  nearly  the  direction  of 
the  long  axis  of  the  fore -arm  :  in  such  a  case 
the  entire  force  of  the  weight  would  be  a  dis- 
placing force  :  and  not  only  would  the  force  of 
the  muscles  co-operate  with  the  articular  lig- 
aments, but  the  force  of  the  weiofht  would 
antao^onize  the  articular  lio-aments  as  well  as  the 
force  of  the  muscles,  unless  the  lifting  muscles 
had  the  power  to  keep  the  weight  from  making 
tension  on  the  ligaments. 

(D.)    A  case  might   also  arise  in  which    the 


THE    PRINCIPLES    OF    MVODYNAMICS.  Sq 

entire  force  of  the  weight  would  be  retentive 
without  the  direct  co-operation  of  the  articular 
liofaments,  for  instance,  when  a  wei^^ht  is  lifted 
nearly  directly  upward  by  the  hand,  having  the 
long  axis  of  the  fore -arm  and  hand  in  a  perpen- 
dicular position,  the  hand  being  above  the  fore- 
arm. 

(E.)  When  the  displacing  component  is  greater 
than  the  resistance  of  tJie  articidar  ligaments,  these 
ligaments  mnst  give  way,  and  the  movable  bone 
zuill  be  dislocated. 


THE    HAND    AND    THE    WRIST-JOINT. 

42.  The  hand  can  act  as  a  lever  of  the  first, 
second,  or  third  order. 

(i.)  When  the  hand  is  extended  and  sustains 
a  weiofht,  it  acts  as  a  lever  of  the  first  order. 

(2.)  When  the  heads  of  the  metacarpal  bones 
rest  on  a  solid  surface,  the  hand  acts  as  a  lever 
of  the  second  order. 

(3.)  When  the  hand  is  flexed  and  Hfts  a 
weight,  it  acts  as  a  lever  of  the  third  order. 


6o  THE    PRTNXIPLES    OF    MY(3DYNAMICS. 

43.  The  hand  lever  Is  in  myodynamic  relation 
with  the  bones  of  the  fore-arm  through  the 
muscles  that  span  the  wrist-joint.  These  mus- 
cles may  be  divided  into  four  groups. 

(i.)  i\n  anterior  group  <zox\X2Xx\\x\'g  the  palmaris 
longus,  the  fleA'or  carpi  radialis.  the  flexor  carpi 
ulnaris,  the  flexor  sublimis  digitorum.  the  flexor 
longus  pollicis,  and  the  flexor  profundus  digi- 
torum. 

(2.)  An  oittward group  containing  the  extensor 
carpi  radialis  longior,  the  extensor  carpi  radialis 
brevior,  the  three  extensors  of  the  thumb,  and 
the  flexor  carpi  radialis. 

(3.)  A  posterior  group  containing  the  two 
carpo-radial  extensors,  the  extensor  carpi  ul- 
naris, the  extensor  conmumis  digitorum,  the  ex- 
tensor minimi  digiti,  the  extensor  indicis,  and 
the  three  extensors  of  the  thumb. 

(4.)  An  inzuard  group  containing  the  flexor 
carpi  ulnaris  and  the  extensor  carpi  ulnaris. 

(5.)  The  muscles  of  the  anterior  group  flex, 
those  of  the  outward  group  abduct,  those  of  the 
posterior  group  extend,  and  those  of  the  inward 
group  addttct  the  hand. 


THE    PRINXIPLES    OF    MVODYNAMICS.  6 1 

44.  Some  of  the  dynamic  relations  of  the 
wrist-joint  are  Important. 

(i.)  There  Is  generally  an  inward  and  for- 
ward obliquity  of  the  floor  of  the  base  of  the 
radius  of  lo  or  i5  degrees. 

(2.)  The  tendons  of  the  fle-xors  meet  the 
plane  of  the  floor  of  the  base  of  the  radius  at 
an  ano-le  somewhat  orreater  than  lo  or  i5  de- 
grees. 

(3.)  The  tendons  of  the  extensors  of  the  thumb 
meet  the  plane  of  the  floor  of  the  base  of  the 
radius  at  an  angle  greater  than  lo  or  i5  de- 
grees. 

(4.)  The  tendons  of  the  rest  of  the  extensors 
of  the  hand  meet  the  plane  of  the  floor  of  the 
base  of  the  radius  at  an  angle  somewhat  less 
than  10  or  i5  degrees. 

(5.)  The  tendons  that  span  the  zvrist-joint  act 
as  ligaments.  They  come  against  the  base  of 
the  radius  and  help  prevent  displacements  of 
the  hand. 

(6.)  The  carpus  moves  forwards,  outwards, 
backwards,  and  Inwards  In  the  concavity  of  the 
base  of  the  radius. 


62  THE    PRINCIPLES    OF    MVODYNAMICS. 

45.  The  extent  of  the  motions  of  the  hand 
may  be  illustrated  by  the  following  measure- 
ments of  a  case  : 

(i.)  From  the  long  axis  of  the  fore-arm  the 
hand  \\2iS  flexed  70  degrees. 

(2.)  From  the  long  axis  of  the  fore -arm  the 
hand  was  abditded  10  decrees. 

(3.)  From  the  long  axis  of  the  fore-arm  the 
hand  was  extended  'j^  degrees. 

(4.)  From  the  long  axis  of  the  fore -arm  the 
hand  was  adduded  35  degrees. 

(5.)  With  external  force  the  hand  can  be 
moved  further  in  any  direction  than  has  just 
been  noted. 

46.  In  a  certain  case  the  dimensions  of  the 
hand-lever  were  as  follows  : 

(i.)  The  entire  length  was  4  inches — the  dis- 
tance from  the  heads  of  the  metacarpal  bones  to 
the  base  of  the  radius. 

(2.)  The  distance  from  the  heads  of  the  meta- 
carpal bones  to  the  point  where  the  force  in  the 
continuity  of  the  lever  acts  was  3  inches. 

(3.)  The  distance  from  the  point  where  the 


THE    PRINCIPLES    OF    MYODYNAMICS.  63 

force  in  the  continuity  of  the  lever  acts  to  the 
carpal  end  of  the  lever  was  i  inch. 

47.  In  the  case  in  which  the  dimensions  of 
the  hand-lever  were  measured,  let  the  entire 
weight  of  the  body — i5o  pounds — rest  on  a  re- 
sisting surface  through  the  heads  of  the  meta- 
carpal bones.  The  hand  is  now  a  lever  of  the 
second  order  :  The  weight  of  the  body  eqicals 
the  pressure  of  the  carpus  on  the  base  of  the 
radius. — We  have  two  equations,  and  two  un- 
known quantities  : 

i5o+P=.W  .         .         .         .  (i.) 

4xP=3xW     ....         (2.) 
From  these  equations  we  find  the  values  of 
P  and  W  : 

P=45o  pounds       ....         (3.) 
W=:6oo  pounds     ....  (4.) 

In  this  case  :  (i.)  When  the  hand  is  extended 
the  fulcrum  is  on  the  palmar  aspect  of  the  heads 
of  the  metacarpal  bones  ;  (2.)  When  the  hand 
is  flexed  the  fulcrum  is  on  the  dorsal  aspect  of 
the  heads  of  the  metacarpal  bones. 

48.  In  the  next  place,  let  the  hand-lever,  as  a 
lever  of  the  third  order,  having  the  same  dimen- 


64  THE    PRINCIPLES    OF    MVODYXAMICS. 


THE    PRINCIPLES    OF    MYODYXAMICS.  65 

sions  as  before,  be  flexed,  and  lift  a  weiorht  of 

5o  Pounds.    Then  we  shall  have  two  equations  : 

P=:4XW=200       .  .  .  .  (i.) 

F=.P-\V=:i5o       ....         (2.) 

(i.)  The  force  of  the  contracting  muscles 
equals  200  pounds. 

(2.)  The  pressure  of  the  carpus  on  the  base 
of  the  radius  equals  i5o  pounds. 

49.  The  myodynamic  relations  of  the  wrist- 
joint  are  conservative — even  on  the  application 
of  considerable  external  force  : — 

(i.)  The  hand  is  very  rarely  dislocated  : 

(2.)  The  base  of  the  radius  is  frequently 
broken  : 

(3.)  These  conclusions  agree  with  the  practical 
observations  of  surgeons. — 

Examples: — In  Figs.  18  and  19,  H=the 
hand  ;  C=the  carpus  ;  R  a=the  radius  ;  U=the 
ulna ;  f-a=the  fore-arm  ;  f-m=the  flexors  011 
the  fore-arm  ;  e-m=the  extensors  on  the  fore- 
arm ;  f-c-R=the  flexor  carpi  radialis  ;  P  0= 
the  plane  of  the  base  of  the  radius  ;  R=a 
retentive  component ;  and  d=a  displacing  com- 
ponent :      Fig.    18    is   an    outside    view    of  the 


66  THE    TRINCIPLES    OF    MVODYNAMICS. 


THE    PRINCIPLES    OF    MYODYXAMICS.  67 

wrist-joint  ;  Fig.  19  is  a  front  view  of  the 
wrist-joint: — By  a  simple  inspection  of  the 
figures  it  can  be  seen  how  the  rcteiitive  compon- 
ents of  the  muscles  that  span  the  wrist-joint  pre- 
ponderate over  the  displacing  components  of 
these  muscles. 

5o.  The  retentive  pressure  of  the  muscles  on 
the  fore-arm  is  expended  on  the  base  of  the 
radius.  And  sometimes  in  old  age  and  after 
contusions  and  sprains  the  base  of  the  radius  is 
shortened  and  otherwise  deformed  by  interstitial 
absorption.  A  condition  of  this  kind  will  more 
or  less  resemble  the  results  of  a  fracture  of  the 
base  of  the  radius. 


THE    RADIUS    AXD    THE    RADIO-ULXAR-JOIXT. 

5 1.  The  base  of  the  radius  turns  about  the 
head  of  the  ulna  as  a  wheel  turns  about  its  axle, 
while  the  head  of  the  radius  turns  on  its  own 
axis  in  a  concavity  of  the  base  of  the  ulna.  The 
radius  turns  inward  and  outward. 

(i.)  The  inward  turn  of  the  radius  is  called 
in-rotation. 


68  THE    PRINCIPLES    OF    MYODYNAMICS. 

(2.)  The  outward  turn  of  the  radius  is  called 
out -rotation, 

(3.)  The  arc  of  rotation  of  the  radius  is  about 
180  degrees. 

52.  When  the  fore -arm  is  completely  ex- 
tended the  long-  axis  of  the  ulna  is  nearly  on  a 
line  with  the  long  axis  of  the  humerus. 

(i.)  Completely  sitpinatc  the  radius  and  the 
lontr  axis  of  the  fore-arm  and  the  lonor  axis  of 
the  humerus  will  meet  forming  an  obtuse  angle 
on  the  outside. 

(2.)  Completely  pronate  the  radius  and  the 
long  axis  of  the  fore-arm  will  be  quite  on  a  line 
with  the  lonor  axis  of  the  humerus. 

(3.)  During  the  rotation  of  the  radius  a  line 
drawn  from  the  point  of  the  styloid  process  to 
the  upper  end  of  the  axis  of  the  radial  head  will 
describe  one-half  of  a  cone  :  This  may  be  called 
the  radial  cone. 

^'iy.  The  following  dimensions  of  the  radial 
cone  were  taken  in  a  given  case  : — 

(i.)  The  radius  of  the  base  of  the  radial  cone 
was  about  one  and  three-fourths  inches  in 
lenorth. 


THE    PRINCIPLES    OF    MYODYNAMICS.  69 

(2.)  The  altitude  of  the  radial  cone  was  about 
nine  and  one-fourth  inches  in  length. 

(3.)  The  slant  height  of  the  radial  cone  was 
about  nine  and  two-fourths  inches  in  lenofth. 

(4.)  The  axis  of  the  radial  cone  prolonged 
downward  will  nearly  coincide  with  the  long  axis 
of  the  rinor-finorer. 

o  o 

54.  The  following  dimensions  of  the  quad- 
rangle of  the  fore-arm  were  taken  in  the  same 
case  : — 

(i.)  One  side  of  the  quadrangle  was  about 
one  and  three-fourths  inches  in  length. 

(2.)  The  other  side  of  the  quadrangle  was 
about  ten  inches  in  leno^th. 

(3.)  The  proximal  end  of  the  quadrangle  of 
the  fore-arm  is  measured  on  the  lower  end  of 
the  humerus. 

55.  Flex  the  fore-arm  so  that  its  longf  axis 
meets  the  lono-  axis  of  the  humerus  at  rieht 
angles,  and  put  the  fore-arm  in  a  position  of 
mid-rotation  : — 

(i.)  The  plane  of  the  bones  of  the  fore-arm 
will  be  nearly  vertical, 

(2.)  The  long  axis  of  the  humerus  will  meet 


/O  THE    PRINXIPLES    OE    MVODYNAMICS. 

a  vertical  line  in  the  plane  of  the  fore-arm  at  an 
angle  of  about  135  degrees. 

56.  The  muscles  that  rotate  the  fore-arm  are 
divided  into  two  groups  : — The  pronators  and 
the  supinators : — 

(i.)  The  pronators  of  the  fore-arm  are — the 
pronator  qiiadratiis,  the  pronator  radii  teres,  the 
supinator  longus,  the  biceps  brachii,  the  flexor 
carpi  radialis,  the  flexor  sublimis  digitorum,  and 
the  flexor  profundus  digitorum. 

(2.)  The  siLpi7iators  of  the  fore-arm  are — The 
supinator  brevis,  the  extensor  ossis  nietacarpi  pol- 
licis,  the  extensor  prinii  internodii  pollicis,  the 
extensor  secundi  internodii  pollicis,  the  supinator 
longus,  the  extensor  carpi  radialis  langior,  and 
the  biceps  brachii. 

57.  The  supinator  longus  has  three  func- 
tions : — 

(i.)  It  flexes  the  radius;  (2.)  it  pronates  the 
radius  during  the  first  part  of  in-rotation ;  (3.)  it 
supinates  the  radius  during  the  first  part  of  out- 
rotation. 

58.  A  muscle  whose  lonof  axis  runs  in  the 
direction  of  the  diagonal  of  the   quadrangle  of 


THE    PRINCIPLES    OF    MVODYXAMICS.  71 


^2  THE    PRINCIPLES    OF    MYODYNAMICS. 

the  fore-arm  will  have  its  force  resolved  \x\\.ofo2ir 
components  in  so  far  as  the  elbow-joint  is  con- 
cerned, and  into  th^ee  components  in  so  far  as  the 
radio-ulnar-joint  is  concerned  : — See  Fig.  20. 

(i.)  Let  P  be  the  contracting  muscle,  E  its 
origin,  and  m  its  insertion  :  Construct  on  m  P  E 
the  lone  axis  of  the  contracting  muscle,  a  rec- 
tangular  parallelo  pipedon.  The  force  of  P  will 
evidently  be  resolved  into  the  components  a,  b, 
and  c: — (i)  The  moving  component  will  be  a  ; 
(ii)  the  retentive  component  will  be  b  in  the 
case  of  the  radius,  and  a  in  case  of  the  ulna  ; 
(iii)  the  displacing  component  will  be  a  in  the 
case  of  the  radius,  and  b  in  the  case  of  the 
ulna  ;  (iv)  and  the  lateral  displacing  component 
will  be  c. 

(2.)  The  component  a  equals  the  component 
e,  and  the  component  c  equals  the  component 
d  : — But  e  is  a  component  making  in-rotation  of 
the  radius,  and  may  be  called  (i)  a  rotating  com- 
ponent ;  and  this  component  tends  to  displace 
the  radius  from  the  ulna,  and  may  be  called  the 
(ii)  rotating  displacing  co7nponent ;  while  the 
component  d  tends  to  hold    the  radius  against 


THE    PRI^'CIPLES    OF    MVODVXAMICS. 


/3 


the  ulna,  and  may  be  called  the  (iii)  lateral  re- 
tentive covtponent. 

59.  The  resultant  of  e  and  d  is  marked  3  in 
the  figure  : — and  the  resultant  of  3  and  b,  or 
of  e,  d,  and  b  is  marked  5  in  the  figure  :  Hence, 
the  force  of  P,  that  is,  5,  is  resolved  into  3.  and 
4  :  So  that  the  lines  3  and  4  represent  the  two 
general  rectangular  components. 

60.  The  lateral  retentive  components  of  the 
inrotators  of  the  fore-arm  are  greater  than  the 
rotating  components  :  Hence  these  muscles  act 
in  a  conservative  manner  in  so  far  as  the  radio- 
ulnar joint  is  concerned. 

61.  A  special  group  of  muscles  supinating  the 
radius  comprises — the  supinator  brevis,  the  ex- 
tensor ossis  metacarpi  pollicis,  the  extensor 
primi  internodii  pollicis,  and  the  extensor  se- 
cundi  internodii  pollicis.  These  muscles  run 
obliquely  across  the  quadrangle  of  the  fore-arm, 
and  have — 

(i.)  Retentive  components  acting  on  the 
radius  in  a  proximal  direction. 

(2.)  Lateral  components  acting  from  without 
inward. 


74  THE    PRINCIPLES    OF    MYODYNAMICS. 

(3.)  The  lateral  components  will  be  resolved 
into  rotating  and  retejitive  components  : — (a) 
The  retentive  components  will  hold  the  radius 
against  the  ulna  : — (b.)  The  rotating  components 
will  rotate  the  radius. 

(4.)  The  lateral  retentive  components  of  the 
special  supinators  are  greater  than  the  rotating 
components  of  these  muscles. 

(5)  Hence,  the  forces  of  the  special  outro- 
tators  of  the  fore-arm  act  in  a  conservative 
manner  in  so  far  as  the  radio-ulnar  joint  is  con- 
cerned. 

62.  The  function  of  the  s2ipinator  longiis  de- 
serves special  attentioji. 

(i.)  In  the  first  place,  when  the  fore-arm  is 
completely  extended,  the  origin  of  the  supina- 
tor longus  will  be  nearly  in  a  line  with  the  axis 
of  the  radial  cone,  and  as  the  radius  rotates  in- 
and-out,  the  long  axis  of  the  supinator  longus 
will  coincide  nearly  with  the  slant-height  of  the 
radial  cone  : — So  that,  under  the  circnmstances, 
the  snpi7iator  longus  will  not  be  a  strong  rotator. 

(2.)  In  the  second  place,  flex  the  fore-arm 
until    it  makes  a  rio^ht  anMe  with  the  arm,  and 


THE    PRINCIPLES    OF    MYODYNAMICS.  7:) 

let  the  ulna  be  fixed  in  this  position  :  As  the 
radius  rotates  completely  in-and-out,  the  inser- 
tion of  the  supinator  longus  will  describe  a 
semi-circle,  and  it  is  evident  that  the  top  of  this 
semi-circle  will  be  nearer  the  origin  of  the 
supinator  longus  than  any  other  points.  Hence, 
under  the  circumstances,  the  supinator  longus 
will  shorten  from  complete  supination  to  mid- 
rotation,  and  elongate  from  mid-rotation  to  com- 
plete pronation  ;  but  all  the  time  the  supinator 
longus  will  act  as  a  flexor  of  the  forearm. 

(3.)  In  the  third  place,  the  moving  component 
of  the  supinator  longus  tends  to  displace  the 
radius  from  the  ulna — especially  the  base  of  the 
radius  from  the  head  of  the  ulna. 

63.  The  supinator  longus  is  properly  a  flexor 
of  the  radius — even  as  much  as  the  biceps 
brachii  : 

(1.)  In  a  given  case,  the  length  of  the  radius 
was  nine  inches,  and  the  length  of  the  power- 
arm  of  the  bony  lever  was  one  and  one-fourth 
inches — the  power  being  the  biceps  brachii. 
The  force  of  the  biceps  brachii  must  be  360 
pounds  to  lift  5o  pounds. 


76  THE    PRINCIPLES    OF    MYODYNAMICS. 

(2.)  In  the  same  case,  the  distance  of  the 
origin  of  the  supinator  longus  from  the  head  of 
the  radius  was  about  four  inches  :  Let  the 
dynamic  triangle  be  right  angled  at  the  joint — 
the  force  of  the  sitpinator  longtis  must  be  only  122 
pounds  to  lift  50  pounds.  Now,  if  we  suppose 
that,  on  the  average,  the  supinator  longus  can 
lift  twice  as  much  as  the  biceps  brachii,  then  it 
will  follow  that  the  supinator  longus  can  lift  as 
much,  on  the  average,  as  the  biceps  brachii  and 
the  brachialis  anticus.     See  Fig.  2 1 . 

(3.)  But,  in  a  given  case,  according  to  the 
above  calculations,  the  supinator  longus  can  lift 
nearly  three  times  as  much  as  the  biceps  brachii. 
The  weight  is  lifted  by  means  of  the  hand  in 
each  instance. 

(4.)  It  must  be  remembered,  however,  that 
the  biceps  brachii  has  a  greater  transverse  sec- 
tion than  the  supinator  longus,  and,  in  this 
respect,  can  exert  more  power  than  the  latter 
muscle.  The  biceps  brachii  makes  motion — the 
supinator  longus  applies  force. 


THE    PRINCIPLES    OF    MYODYNAMICS.  'jl 


f-au 


yS  THE    PRINCIPLES    OF    MVODVXAMICS. 


THE    FORE-ARM    AND    THE    ELBOW-JOINT. 

64.  The  bones  of  the  fore-arm  can  act  as  a 
lever  of  the  first,  second,  or  third  order. 

(i.)  Elevate  the  arm  and  lift  a  weight  with  the 
hand  ;  the  triceps  brachii  will  move  the  ulna  and 
carry  along  the  radius,  making  the  bones  of  the 
fore-arm  a  lever  of  the  first  order. 

(2.)  Rest  the  hand  on  a  resisting  surface,  as  a 
fulcrum,  and  the  bones  of  the  fore  Avill  be  a  lever 
of  the  second  order,  whose  power  comes  from 
the  triceps  brachii.  It  is  evident  that  the  me- 
chanical principles  involved  in  these  two  cases 
are  similar,  the  weight  and  the  fulcrum  only 
changing  places,  while  the  power  continues  to 
act  at  the  same  point. 

(3.)  Under  the  action  of  the  biceps  brachii 
and  the  brachialis  anticus,  the  bones  of  the  fore- 
arm act  as  a  lever  of  the  third  order. 

65.  The  fore-arm  has  three  motions  : 

(i.)  The  rotation  of  the  fore-arm  has  already 
been  described. 

(2.)  The  fore-arm  moves  forward  and  back- 


THE    PRINCIPLES    OF    MVODVXAMICS.  79 

ward  about  1 30  degrees  :    Flexion  and  extension. 

(3.)  In  early  life  the  fore -arm  moves  outward 
and  inward,  sometimes  as  much  as  i5  deo^rees  : 
Abduction  and  adduction — a  lateral  ginglymus. 

66.  The  elbow  joint  has  two  essential  parts  : 

A.  (i.)  The  ulno-numeral  articulation  is  a 
hinge -joint,  and  the  greater  sigmoid  cavity  of 
the  ulna  always  embraces  the  trochlea  of  the 
humerus. 

(2.)  The  radio-humeral  articulation  is  a  hinge- 
joint  and  a  ball-and-socket-joint,  having  a  shallow 
socket  in  the  head  of  the  movable  bone,  and  a 
small  head  on  the  fixed  bone.  The  radius  has 
only  semi-circiimductio7i,  which,  in  so  far  as  the 
ulna  is  concerned,  is  rotation. 

B.  (i.)  The  distal  end  of  the  humerus,  from 
the  grove  of  the  trochlea  outward,  is  nearly 
transverse  to  the  long  axis  of  the  humerus. 

(2.)  The  lip  of  the  trochlea  projects  down- 
ward about  30  degrees  from  the  transverse  di- 
ameter of  the  condyles  of  the  humerus.  This  lip 
antagonizes  the  inward  lateral  displacing  com- 
ponents of  such  muscles  as  are  attached  to  the 
internal  condyle,  and  run  obliquely  downward 


8o  THE    PRINCIPLES    OF    MVODVXAMICS. 

and  outward  across  the  quadrangle  of  the  fore- 
arm. 

67.  The  muscles  that  span  the  elbow  joint 
may  be  divided  into  three  groups  : 

(i.)  The  r^/<^/6>r5"  of  the  fore-arm  have  already 
been  enumerated. 

(2.)  The  flexors  of  the  fore-arm  are  :  (i.)  The 
supinator  longus,  the  biceps  brachii,  and  the 
brachialis  anticus.  (ii.)  The  pronator  radii  teres, 
and  the  flexors  07i  the  fore -arm,  and  the  long 
carpo-radial  extensor. 

(3.)  The  extensors  of  the  fore-arm  are  :  (i) 
The  triceps  brachii  and  the  supinator  brevis.  (ii) 
The  extensors  on  the  fore-arm. 

(a.)  The  flexors  on  the  fore-arm  and  the  ex- 
tensors on  the  fore-arm  have  indirect  moving 
components  acting  on  the  radius  and  ulna. 

(b.)  A  muscle  that  spans  two  joints  can  make 
the  intermediate  bone  a  fixed  bone,  and  the 
adjacent  bones  movable  bones.  One  of  the 
adjacent  bones  may  be  the  fixed  bone,  and  the 
other  two  movable  bones. 

68.  In  the  next  place,  the  following  proposi- 
tion can  be  proved  : 


THE    PRIXCTPLKS    OI--    MVODVNAMICS.  8 1 

The  muscles  that  directly  and  indirectly  flex  the 
forearm  are  more  pozuerftd  than  those  zuhich  di- 
rectly and  indirectly  extend  the  forearm. 

I  made  transverse  sections  of  the  upper  limb 
of  a  muscular  male,  whose  cadaver  had  been 
preserved  for  dissection.  First  :  I  made  a 
transverse  section  of  the  fore  arm  three  inches 
above  the  wrist  joint  :  (i)  The  section  of  the 
muscles  on  the  dorsal  aspect  showed  an  area  of 
about  one  and  one-eighth  square  inches  ;  and 
(2),  the  section  of  the  muscles  on  the  palmar 
aspect  showed  an  area  of  about  two  square 
inches.  Second:  I  made  a  transverse  section  of 
the  fore- arm  four  inches  below  the  elbow  joint  : 

( 1 )  The  section  of  the  muscles  on  the  dorsal 
aspect  showed  an  area  of  about  two  square  inches  ; 

(2)  the  section  of  the  muscles  on  the  palmar 
aspect  showed  an  area  of  about  four  and  one- 
half  square  inches.  Third  :  I  made  a  transverse 
section  of  the  middle  of  the  arm  :  ( i )  The  sec- 
tion of  the  muscles  on  the  dorsal  aspect  showed 
an  area  of  about  two  and  one -half  square  inches  ; 
and  (2),  the  section  of  the  muscles  on  the  palmar 


82  THE    PRINCIPLES    OF    MYODVXAMICS. 

aspect  showed  an  area  of  about  three  and  one- 
half  square  inches. 

69.  From  the  above  measurements  may  be 
drawn  the  following  conclusions — namely, 

I.  In  the  lower  part  of  the  forearm  the  volume 
of  the  muscles  on  the  posterior  aspect  is  to  the 
volume  of  the  muscles  on  the  palmar  aspect  as 
9  :    16. 

II.  In  the  upper  part  of  the  fore-arm  the  vol- 
ume of  the  muscles  on  the  posterior  aspect  is 
to  the  volume  of  the  muscles  on  the  palmar 
aspect  as  4  :   9. 

III.  In  the  middle  of  the  arm,  the  volume  of 
the  muscles  on  the  posterior  aspect  is  to  the 
volume  of  the  muscles  on  the  palmar  aspect  as 
5  :   7. 

IV.  In  ofeneral,  the  volume  of  the  muscles  on 
the  palmer  aspect  of  the  upper  limb  is  nearly 
twice  as  orreat  as  the  volume  of  the  muscles  on 
the  posterior  aspect  of  the  same  limb.  The 
ratio  may  vary  from  the  above  estimate. 

JO.  It  is  well  known  that  the  proper  use  of  a 
muscle  will  increase  its  power  up  to  a  certain 


THE    PRINXIPLES    OF    MVODVXAMICS.  8 


J 


point.  And  the  muscles  on  the  pahiiar  aspect 
of  the  upper  Hmb  are  used  more  than  the  mus- 
cles on  the  dorsal  aspect  of  the  upper  limb, 
because  the  former  are  habitually  made  to  over- 
come greater  resisting  forces  than  the  latter. 
Hence.  tJie  flexors  of  the  icpper  limb  are  more 
paiuerfjil  than  the  extensors. 

71.  (a.)  When  the  radius  is  completely  ex- 
tended the  radio-humeral  joint  is  conserved  by  : 

(I.)  The  retentive  components  of  the  supina- 
tor longus  and  the  biceps  brachii. 

(2,)  The  retentive  components  of  the  pronator 
radii  teres  and  the  supinator  brevis. 

(b.)  And  the  superior  radio-ulnar  joint  is  con- 
served by  the  lateral  retentive  components  of 
the  pronator  radii  teres  and  the  supinator  brevis. 

72.  W  hen  the  radius  is  semiflexed  : 

(i.)  The  displacing  component  of  the  biceps 
brachii  will  be  large. 

(2.)  The  displacing  component  of  the  pronator 
radii  teres  will  be  relatively  small. 

(3.)  The  retentive  component  of  the  supinator 
longus  will  be  large,  and  its  moving  component 
will  not  act  as  a  displacing  component. 


84  THE    PRINCIPLES    OF    M VODVXAMICS. 

(4.)  The  special  ab-diictors  of  the  hand — that 
is,  the  two  carpo-7^adial  extensors — will  have  large 
retentive  components  f — 

So  that  the  radio-humeral  joint  will  be  in  a 
comparative  condition  of  conservatism. 

73.  When  the  radius  is  completely  flexed  : 
(i.)  The  greater   parts  of  the    forces   of  the 

biceps  brachii  and  of  the  brachialis  anticus  will 
be  displacing  components. 

(2.)  But  these  displacing  components  will  be 
antagonized  by  parts  of  the  forces  of  the  supi- 
nator longus,  the  supinator  brevis,  the  special 
abductors  of  the  hand,  and  other  muscles  that 
span  the  elbov^-joint  and  have  their  origin  on 
the  lower  end  of  the  humerus.  In  this  respect, 
the  radio-humeral  joint  is  in  a  conservative 
relation  ;  and  the  ulno-humeral  joint  has  con- 
servative myodynamic  relations  in  any  position 
of  the  fore-arm  except  one^ — namely, 

74.  When  the  fore -arm  is  flexed  about  2  5 
degrees.  The  co-operation  of  a  resisting  force 
against  the  palm  of  the  hand  and  the  displacuig 
components  of  the  muscles  enumerated  below 
will  dislocate  the  ulna  and  the  radius  :— 


THE    I^RIXCIPLES    OF    MYODYNAMICS.  85 

The  triceps  brachii,  the  flexors  on  the  fore-arm, 
the  extensors  on  the  fore-arm,  the  supinator 
longus,  and  the  pronator  radii  teres.  It  is  evi- 
dent that  the  supination  of  the  radius  aids  in 
causing"  this  dislocation. 

"i^.  This  dislocation  is  the  more  possible,  be- 
cause the  shaft  of  the  ulna  is  almost  wholly,  as 
it  were,  back  of  the  condyles  of  the  humerus, 
and  because  the  coronoid  process,  at  times,  is 
somewhat  readily  lifted  from  the  trochlea  by  the 
impact  of  violence  and  the  force  of  displacing 
components  : — when  the  radius  is  carried  with 
the  ulna,  assisted  by  displacing  muscular  com- 
ponents. 

76.  In  regard  to  the  stability  of  the  elbow- 
joint,  the  following  statements  may  be  made  : 

(i.)  When  the  fore-arm  is  completely  ex- 
tended, the  radio-humeral  joint  is  in  a  condition 
of  maximum  stability. 

(2.)  When  the  fore-arm  and  the  arm  meet  at 
a  right  angle,  the  ulno-humeral  joint  is  in  a 
condition  of  maximum  stability. 

(3.)  The  elbow-joint  has  less  stability  than  the 
wrist-joint. 


86  THE    PRINCIPLES    OF    MYODVNAMICS. 

']'] .   In  a  given  case,  it  was  found  that, 

(i.)  During  flexion  of  the  fore-arm,  the  inser- 
tion of  the  biceps  brachii  approached  its  origin 
about  three  inches. 

(2.)  During  flexion  of  the  fore-arm,  the  inser- 
tion of  the  supinator  longus  approached  its  origin 
about  seven  inches. 

(3.)  This  shows  that  the  supinator  longus  has 
more  intrinsic  motion  than  the  biceps  brachii : 
It  can  contract  more. 

']'^.  In  an)'  case,  when  the  biceps  brachii  moves 
a  weiorht  in  the  hand,  the  weicrht  moves  as  much 
further  than  the  bicipital  tubercle  of  the  radius  as 
the  weight-arm  of  the  bony  lever  is  longer  than 
the  power-arm.  See  Fig.  2 1 .  The  proportion 
is  as  follows  :  \\'-F  :  P-F  :  :  D  :  d  :  In  which 
D  =  the  distance  which  W  moves,  and  d  ^==z. 
the  distance  which  P  moves. 

(i.)  While  the  weight  is  moving,  it  can  do 
more  work  at  any  one  moment  than  the  power 
of  the  biceps  ;  because  the  energy  of  the  weight 
is  proportional  to  the  square  of  its  velocity  ;  while 
the  energy  of  the  power  is  proportional  to  the 
square  of  its  velocity  ;  and  because  the  excess 


THE    PRINCIPLES    OF    MYODYNAMICS.  Sy 

of  energy  in  the  biceps  necessary  to  move  the 
weight  acciumdates  in  the  weight. 

(2.)  The  simple  fact  is,  that  the  molecular 
motion  of  the  biceps  is  correlated  into  the  molar 
motion  of  the  radius  and  the  weight;  and,  as 
molar  motion  represents  energy,  there  is  no 
loss  of  muscular  force. 


THE    HUMERUS    AND    THE    SHOULDER-JOINT. 

79.  The  humerus  may  act  as  a  lever  of  the 
first,  second,  or  third  order  : 

(1.)  The  transverse  diameter  of  the  tuberoci- 
ties  of  the  humerus  acts  as  a  lever  of  the  first 
order  when  moved  by  the  rotators  The  in- 
rotators  of  the  arm  may  be  the  power,  and  the 
out-rotators  may  be  the  weight. 

(2  )  The  supraspinatus  can  act  on  the  humerus 
so  as  to  make  it  a  lever  of  the  second  order, 
when  the  weight  of  the  body  is  put  more  or  less 
on  the  head  of  the  humerus,  and  consequently 
on  the  outside  of  the  elbow. 


S8  THE    PRINCIPLES    OF    MY(M)VXAMICS. 

(3.)  The  pectoralis  major,  for  instance,  makes 
the  humerus  a  lever  of  the  third  order. 

80.  The  motions  of  the  arm  may  be  arranged 
under  four  heads  : 

(I.)  The  arm  can  move  inward  and  outward 
about  90  degrees — addiictioji  and  abduction. 

(2.)  The  arm  can  move  backward  and  forward 
about  135  degrees — extension  and  flexion. 

(3.)  The  arm  can  rotate  inward  and  outward 
about  90  degrees — in-rotation  and  out-rotation. 

(4.)  The  arm  can  move  around  to  the  right 
or  to  the  left  360  degrees — circuniduction. 

81.  The  muscles  that  move  the  arm  may  be 
grouped  according  to  the  direction  of  these 
motions  : 

(I.)  The  addicctors  of  the  arm  are  the  pector- 
alis major,  the  teres  major,  the  teres  minor,  the 
subscapularis.  the  infraspinatus,  and  the  latissi- 
mus  dorsi. 

(2.)  The  flexors  of  the  arm  are  the  biceps 
brachii,  the  coraco-brachialis,  the  pectoralis  ma- 
jor, and  the  anterior  part  of  the  deltoid. 

(3.)  The  abductors  of  the  arm  are  the  deltoid 
and  the  supraspinatus. 


THE    PRINCIPLES    OF    MYODYNAMICS.  89 

(4.)  The  extensoi^s  of  the  arm  are  the  triceps 
brachli,  the  posterior  part  of  the  deltoid,  the 
infraspinatus,  the  teres  minor,  the  teres  major, 
the  subscapularis,  and  the  latissimus  dorsi. 

(5.)  The  oiLt-7'otators  of  the  arm  are  the  teres 
minor,  the  infraspinatus,  the  supraspinatus,  and 
the  posterior  part  of  the  deltoid. 

(6.)  The  ill-rotators  of  the  arm  are  the  sub- 
scapularis, the  pectoralis  major,  the  anterior  part 
of  the  deltoid,  the  teres  major,  and  the  latissi- 
mus dorsi. 

(7,)  The  circiimdiictors  of  the  arm  comprise, 
in  one  way  or  another,  all  the  muscles  that  span 
the  shoulder-joint  and  move  the  arm. 

82.  In  an  average-sized  specimen  of  an  adult 
humerus,  the  following  measurements  were 
made  : 

(i.)  The  entire  length  was  about  13  inches. 

(2.)  The  length  of  the  anatomical  neck  was 
about  three-eighths  of  an  inch. 

(3.)  The  depth  of  the  head  was  about  five- 
eighths  of  an  inch. 

(4.)  The  diameter  of  the  head  was  about  one 
and  three-eighths  inches. 


90  THE    PRIN'CIPLES    OF    MYODYNAMICS. 

(5.)  The  long  axis  of  the  humerus  met  the 
plane  of  the  base  of  the  head  of  the  humerus  at 
an  angle  of  about  46  degrees. 

S^.  In  the  same  case,  the  following  observa- 
tions were  made  : 

(a.)  The  external  condyle  and  the  greater 
tuberosity  of  the  humerus  pointed  in  the  same 
direction. 

(b.)  The  internal  condyle  pointed  in  the  same 
direction  as  a  line  on  the  head  of  the  humerus 
midway  between  the  neck  behind  and  the  bi- 
cipital grove  in  front. 

(c.)  The  greatest  conjugate  diameter  of  the 
glenoid  cavity  of  the  scapula  was  about  one  and 
one-fourth  inches  ;  while  the  o-reatest  transverse 
diameter  was  about  one  and  five-eighths  inches. 

(d.)  The  acromion  projected  about  two  inches 
beyond  the  glenoid  cavity. 

(e.)  The  coracoid  process  of  the  scapula  pro- 
jected somewhat  above  and  in  front  of  the  glen- 
oid cavity. 

(f)  The  strong  coraco  -  acromial  ligament 
completed  the  roof  over  the  upper  end  of  the 
humerus. 


THE    PRINCIPLES    OF    MYODYNAMICS.  QI 

84.  In  general,  the  plane  of  the  floor  of  the 
glenoid  cavity  of  the  scapula,  or  its  conjugate 
diameter,  runs  somewhat  backward  from  the 
transverse  plane  of  the  body,  while  the  transverse 
diameter  of  this  cavity  is  nearly  parallel  with  the 
long  axis  of  the  body.  The  surface  of  the  cavity 
looks  foriuard  and  someiuJiat  outward.  The  head 
of  the  humerus  rests  on  this  plane.  Are  there 
any  muscular  components  that  tend  to  displace 
the  head  of  the  humerus  from  the  glenoid  cavity 
of  the  scapula  ?  In  the  meantime,  let  the  mobility 
of  the  scapula,  in  regard  to  the  chest- wall,  be 
kept  in  mind.  And  let  it  also  be  kept  in  mind 
that  the  extensive  motion  of  the  humerus  is 
facilitated  by  the  comparatively  large  articular 
surface  of  its  head  and  by  the  comparatively 
small  articular  surface  of  the  glenoid  cavity  of 
the  scapula. 

85.  Some  of  the  muscles  that  span  the 
shoulder-joint  have  small  displacing  components 
acting  on  the  humerus  : 

(i.)  The  supra  spinatus  has  no  displacing 
components  acting  on  the  humerus.  The  an- 
terior part  and  the  posterior  part  of  the  deltoid 


92  THE    PRINXIPLES    OF    MYODYNAMICS. 

ma}'  have  displacing  components  acting  on  the 
humerus. 

(2.)  The  infraspinatus,  the  subscapularis,  and 
the  teres  minor  may  have  downward  displacing 
components  acting  on  the  humerus. 

(3.)  The  coraco-brachialis  and  the  short  head 
of  the  biceps  brachii  and  the  long  head  of  the 
triceps  brachii  may  have  small  inward  displacing 
components  acting  on  the  humerus. 

86.  Some  of  die  muscles  that  span  the 
shoulder-joint  have  large  displacing  components 
acting  on  the  humerus.  These  are  the  pec- 
toralis  major,  the  teres  major,  and  the  latissimus 
dorsi,  and  the  deltoid  : 

(i.)  The  lower  part  of  the  pectoralis  major 
deserves  special  attention  :  (i)  It  has  a  large 
myodynamic  angle  giving  a  large  displacing 
component  acting  downward.  (ii)  It  pulls 
almost  directly  inward  and  forward,  in  the 
direction  of  the  conjugate  diameter  of  the  glenoid 
cavity,  (iii)  Hence,  the  lower  part  of  the  pec- 
toralis major  has  a  predominant  displacing  com- 
ponent acting  on  the  humerus,  forward,  inward, 
and  downward. 


THE    PRINCIPLES    OF    MYODYNAMICS.  93 

(2.)  For  reasons  that  are  plain,  the  upper  part 
of  the  pectoraHs  major  has  a  large  inward  and 
forward  displacing  component. 

(3.)  The  teres  major  and  the  latissimus  dorsi 
act  nearly  in  the  same  direction  and  have  large 
displacing  components  acting  on  the  humerus, 
downward,  inward,  and  somewhat  backward. 

(4.)  When  the  arm  is  abducted  the  deltoid  will 
have  a  large  displacing  component  acting  on  the 
humerus.  This  results  from  the  fact  that  then 
the  deltoid  pulls  nearly  in  a  line  with  the  con- 
jugate diameter  of  the  glenoid  cavity  of  the 
scapula. 

(5.)  When  the  arm  is  abducted,  the  triceps 
brachii,  by  means  of  its  scapular  origin,  on  ac- 
count of  the  direction  of  the  plane  of  the  glenoid 
cavity,  will  have  a  considerable  displacing  com- 
ponent, acting  forward,  inward,  and  downward. 
on  the  head  of  the  humerus. 

(6.)  He7ice,  it  zuill  appeal'  that  the  shoulder -joints 
tender  some  viyo-dynainic  relations,  is  in  a  condi- 
tion of  remarkable  ifistability . 

(7.)  It  may  be  noted  that  the  arm  can  only  be 
abducted  to  a  right  angle  with  the  body. 


94  rin:   pRiNCii'LEs  of  mvodynamics. 

(8.)  If  the  arm  is  rotated  inward,  the  head  of 
the  humerus  will  turn  under  the  acromion,  and 
the  arm  can  then  be  elevated  so  as  to  come 
more  directly  upward. 

87.  There  are  some  points  of  dynamic  import- 
ance to  add  to  the  facts  above  stated  : 

(i.)  Let  the  elbow  meet  with  a  resisting  sur- 
face— acting  as  3.  power— shducting  the  arm  until 
the  great  tuberosity  of  the  humerus  comes  against 
the  acromion.  Let  the  acromion  be  the  weight 
— ^then  the  resistance  of  the  lower  portion  of  the 
capsule  will  represent  the  fulcrum — which  may 
practically  give  way  and  let  the  head  of  the 
humerus  out  of  the  socket.  T/ie  humerus,  under 
the  circiunstances.  will  be  a  lever  of  the  second 
order — and,  on  account  of  the  leno^th  of  the 
power-arm  and  on  account  of  the  comparative 
shortness  of  the  weight-arm,  will  apply  great 
force  to  the  capsule.  Also  the  prominent  part 
of  the  head  of  the  humerus  will  be  brouofht 
nearer  the  lower  border  of  the  socket,  and  so  be 
more  apt  to  slip  off  into  the  axilla.  And  the 
adductors  of  the  arm  will  be  put  on  the  stretch, 
and.  will  have  their  displacing  components  aug- 


THE    PRINCIPLES    OF    MVODYNAMICS.  gS 

merited,  which  may  be  looked  upon  as  so  much 
weight  to  be  added  to  the  resistance  of  the  acro- 
mion, and  thus  the  displacement  of  the  head  of 
the  humerus  Into  the  axilla  will  be  doubly 
assured — on  the  principle  of  a  lever  of  the 
second  order. — 

(2.)  But,  If  the  displacing  components  of  the 
adductors  and  the  resistance  of  the  acromion  be 
looked  upon  as  the  power,  the  principle  of  a 
lever  of  the  third  order  will  be  involved,  and  the 
fulcrum  will  be  overcome  : — which  Is  the  lower 
and  Internal  portion  of  the  capsular  ligament. 

88.  The  following  problem  may  now  be 
solved  : —  To  deterniiiie.  iii  a  oriveii  case,  the  strain 
on  the  inner  and  lower  portion  of  the  capsule  of 
the  slionlder-joint, — It  will  be  convenient  at 
present  to  consider  the  humerus  a  lever  of  the 
first  order. — 

(i.)  Let  the  power-arm  of  the  lever  be  ten 
Inches  long — the  distance  from  the  elbow-joint  to 
the  acromion.  Let  the  weight- arm  of  the  lexer 
be  two  Inches  long — the  distance  from  the  Inner 
and  lower  portion  of  the  capsule  to  the  acro- 
mion. 


96  THE    PRINCIPLES    OF    MYODYNAMICS. 

In  this  case  the  acromion  will  be  the  fulcrum. 
Let  the  force  applied  to  the  distal  end  of  the 
humerus  equal  lOO  pounds.     Then, 

2  :  lo  r=  loo  :  5oo. 
Hence,  so   far,  the   strain    on    the   capsule    will 
equal  5oo  pounds. 

(2.)  Again,  let  the  power-arm  of  the  lever  be 
ten  inches  long — the  distance  from  the  elbow- 
joint  to  the  insertion  of  the  adductors  of  the 
arm.  Let  the  weight-arm  of  the  lever  be  two 
inches  lone — the  distance  from  the  inner  and 
lower  portion  of  the  capsule  to  the  insertion  of 
the  adductors  of  the  arm.  In  this  case  the  in- 
sertion of  the  adductors  of  the  arm  will  be  the 
fulcrum.  But  the  force,  as  supposed,  applied  to 
the  distal  end  of  the  humerus,  equals  100 
pounds.     Then,  again, 

2  :  10  =  100  :  5oo. 
Hence,  also,  so  far  the  strain  on  the  capsule  will 
equal  5oo  pounds. 

(3.)  Therefore,  under  the  circumstances,  the 
capsule  of  the  shoulder-joint  receives  a  strain 
equal  to  1000  pounds.  And  it  may  happen  that 
the  greater  part  of  this  strain  acts  as  a  displacing 


THE    I'RINXIPLES    OF    MYODYXAMICS.  97 

force,  tending  to  put  the  head  of  the  humerus 
into  the  axilla.  Such  a  result  takes  place  from 
time  to  time. 

91.  The  conformation  and  the  muscular  rela- 
tions of  the  shoulder-joint  above  and  behind  are 
eminently  conservative.  The  conformation  and 
the  muscular  relations  of  the  shoulder-joint 
below  and  partly  in  front  are  notably  non-con- 
servative. These  conclusions  a^ree  with  the 
observations  of  surgeons  in  reo^ard  to  disloca- 
tions  of  the  humerus. 


THE  FOOT  AND  THE  ANKLE-JOINT. 

92.  The  foot  acts  as  a  lever  of  the  second 
order. 

In  the  first  place,  the  following  points  are  to 
be  noted  :  (i.)  The  ankle-joint  is  a  hinge -joint. 
The  foot  has  flexion  downward  and  backward, 
and  extension  upward  and  forward.  (2.)  The 
lateral  motion  of  the  foot  (which  is  principally 
inward)  takes  place  at  the  subastragaloid  joint. 


98  THE    PRI^'CIPLES    OF    MYODYNAMICS. 

(3.)  The  two  malleoli  protect  the  sides  of  the 
ankle-joint.  (4.)  The  extensor  tendons  are  held 
down  by  the  annular  ligament  which,  to  some 
extent,  plays  the  part  of  a  pulley.  (5.)  The 
tendo-Achillis  is  also,  to  a  limited  extent,  held 
down  by  the  annular  ligament.  (6.)  The  astra- 
ealus  elides  and  turns  backward  and  forward 
under  the  tibio-fibular  arch.  (7.)  The  muscles 
of  the  tendo-Achillis  are  especially  used  for 
locomotion  ;  they  are  lifting  muscles.  (8.)  The 
tibialis  posticus,  the  flexor  communis  digitorum, 
the  flexor  longus  pollicis,  the  peroneus  longus, 
and  the  peroneus  brevis  are  especially  used  to 
flex,  abduct,  evert,  and  invert  the  foot. 

93.  (a.)  The  extensors  of  the  foot  are  the 
tibialis  anticus,  the  extensor  longus  pollicis,  the 
extensor  longus  digitorum,  and  the  peroneus 
tertius.  When  the  extensors  lift  the  anterior 
part  of  the  foot,  the  foot  is  a  lever  of  the  third 
order. 

(b.)  The  flexors  of  the  foot  are  the  tibialis 
posticus,  the  flexor  longus  digitorum,  the  flexor 
longus  pollicis,  the  muscles  of  the  tendo-Achillis, 
the  peroneus  longus,  and  the  peroneus  brevis. 


THE    PRI^XIPLES    OF    MYODYXAMICS.  99 

(c.)  The  add7tcto7's  of  the  foot  are  the  tibiaHs 
anticus,  the  tibialis  posticus,  the  flexor  longus 
polHcis,  the  flexor  longus  digitorum.  and  the 
muscles  of  the  tendo-Achillis. 

(d.)  The  abditdors  of  the  foot  are  the  extensor 
communis  digitorum,  the  peroneus  tertius.  the 
peroneus  brevis,  and  the  peroneus  longus. 

94.  It  must  be  kept  in  mind  that  the  gastroc- 
nemius spans  both  the  knee-joint  and  the 
ankle-joint,  arising  from  the  condyles  of  the 
femur  and  inserted  into  the  os  calcis.  The  im- 
portant practical  relation  here  consists  in  the 
fact   that   the   orastrocnemius   can    not   be   com- 

o 

pletely  relaxed  without  flexing  the  leg  on  the 
thigh  and  the  foot  on  the  leg.  This  fact  will 
appear  to  advantage  in  the  reduction  of  disloca- 
tions of  the  foot,  and  in  the  setting  of  broken 
bones  of  the  leer. 

95.  When  the  foot  is  completely  extended, 
the  tendons  of  the  extensors  are  held  down  by 
the  annular  ligament,  and  the  direction  of  trac- 
tion is  downward  and  backward,  so  that  the 
extensors,  under  the  circumstances,  will  have 
strong  displacing  components,  and  at  the  same 


lOO  THE    PRINXIPLES    (3F    MYODVXAMICS. 

time  the  special  flexors  of  the  foot,  as  they  pass 
around  the  malleoH,  will  have  strong  displacing 
components.  The  displacing  components  of 
both  sets  of  muscles  will  act  downward  and 
backward  ;  but  the  muscles  of  the  tendo-x'\chillis, 
during  this  time,  will  have  a  strong  displacing 
component  acting  upward  and  forward,  and 
counteracting  the  displacing  components  of  the 
special  flexors  and  the  extensors  of  the  foot. 

96.  From  the  point  of  complete  extension  the 
foot  can  be  flexed  about  forty-five  degrees.  In 
any  position  of  the  foot,  the  extensors  and 
special  flexors  have  marked  displacing  compon- 
ents actino^  backwards.  But  when  the  tendo- 
Achillis  meets  the  lonof  axis  of  the  foot  at  rio-ht 
anorles,  the  muscles  of  this  tendon  will  not  have 
a  displacing  component ;  all  the  energy  of  the 
muscles  will  be  motor  and  retentive. 

97.  The  displacing  components  of  the  adduct- 
ors of  the  foot  will  tend  to  displace  the  rest  of 
the  foot  from  the  astraofalus  ;  but  the  connec- 
tions  of  the  subastragaloid  joint  are  very  strong, 
and  the  malleolus  internus  will  be  apt  to  give 
way,  under  the  application  of  great  force,  when 


THE    PRINCIPLES    OF    MYODYNAMICS.  lOI 

the   displacement  will  occur  at  the  ankle-joint, 
complicating  a  dislocation  with  a  fracture. 

98.  In  this  place,  it  is  proper  to  determine 
whether  or  not  the  foot  acted  on  by  the  muscles 
of  the  tendo  Achillis  is  a  lever  of  the  second  order. 
Under  the  circumstances,  it  has  already  'been 
shown  by  transposing  terms  that  the  foot  in- 
volves the  principles  of  a  lever  of  the  first  order. 
This  would  be  true  of  any  lever  of  the  second 
order.  In  a  lever  of  the  second  order,  the 
weight  is  soiuewhere  in  the  contifiiiity  of  the  lever, 
the  power  is  at  one  end,  and  the  fulcrum  is  at 
the  other  end  of  the  lever.  The  body  is  cer- 
tainly the  weight  to  be  moved,  and  it  rests  on  the 
continuity  of  the  lever  :  The  muscles  of  the 
tendo-Achillis  act  on  one  end  of  the  lever — this 
is  the  power  ;  the  other  end  of  the  lever  rests 
on  some  resisting  surface — and  this  is  the  ful- 
crum. Here  we  have*  all  the  conditions  of  a 
lever  of  the  second  order.  Hence,  we  conclude 
that  the  foot  is  a  lever  of  the  second  order.  But 
the  weight  to  be  7noved — that  is,  the  weight  of 
the  entire  body — is  only  part  of  the  entire  force 


I02  THE    PRINXIPLES    OF    MYODYNAMICS. 

acting  on  the  continuity  of  the  lever.  The  cfitire 
force  is  the  presstire  exerted  by  the  weight  of 
the  body  on  the  astragalus  under  the  dynamic 
conditions  ;  and  that  is,  as  the  mechanical  facts 
show,  the  weight  of  the  body  plus  the  contract- 
ile force  of  the  active  muscles,  expressed  in 
the  same  units  as  the  weight  of  the  body.  So 
much  for  the  foot-lever. 

99.  It  is  possible  to  verify  this  conclusion  by 
assuming  the  foot  to  be  :  (i-)  ^  lever  of  the 
first  order.     (2.)  A  lever  of  the  second  order  : 

In  a  given  case,  the  distance  from  the  ball  of 
the  foot  to  the  center  of  the  ankle-joint  is  five 
inches,  the  distance  from  the  center  of  the  ankle- 
joint  to  the  insertion  of  the  tendo-Achillis  is 
two  and  one-half  inches,  and  the  weight  of  the 
body  is  1 5o  pounds.  Now,  it  is  evident  that 
there  will  be  a  pressure  of  i5o  pounds  on  the 
ball  of  the  foot  when  the  muscles  of  the  tendo- 
Achillis  lift  the  body  : 

(i.)  The  weight-arm  of  the  lever  X  the 
weight  =  the  power-arm  of  the  lever  X  the 
power  :     That  is — 

2^   X   P   =r:    5    X    I  5o  \         .  ( I .) 


THE    PRINCIPLES    OF    MYODYNAMICS.  IO3 

Hence, 

P   =:    300:      .  .  .  (2.) 

Therefore, 

W  =  45o:  .         .  (3.) 

The  pressure  on  the  tibio-fibular  arch  is  45o 
pounds. 

While  F  =  i5o:    .         .         .         (4.) 

(2.)  Again,  the  weight-arm  of  the  lever  X 
the  weight  =  the  power-arm  of  the  lever  X  the 
power.     That  is  : — 

7i  X  P  =  5  X  W  :        .         (5.) 

Also,  P4-  i5o  =:=  W  :  .         (6.) 

Equating  (5)  and  (6). 

7JxP=rr  5xP-[-75o:         (7.) 

Hence,  P  =  300  :    .         .         .         (8.) 

And  W  =  450  :  .  .  (9.) 

The  result  is  the   same   as   before.     Hejice  the 
foot  is  a  lever  of  the  second  order. 

(3.)  It  appears,  then,  that  the  foot  is  a  lever 
of  the  second  order,  that  the  ordinary  lifting 
force  of  the  muscles  of  the  tendo-Achillis  is  300 
pounds,  and  that  the  habitual  pressure  on  the 
astragalus,  or  on  the  tibio-fibular  arch,  is  from 
i5o  to  460  pounds.     Yet  the  pressure  on  the 


I04         THE    PRINCIPLES    OF    MYODYNAMICS. 

astragalus,  at  times,  must  be  much  greater  than 
45o  pounds.  This  is  especially  the  case  when 
external  violence  is  added  to  muscular  force. 

TOO.  Let  the  foot-lever  be  extended  as  far  as 
possible  ;  from  the  origin  of  the  contracting 
muscles,  taken  c(jllectively,  let  fall  a  perpendicu- 
lar to  the  long  axis  of  the  foot  prolonged 
forward  :  The  distance  from  the  foot  of  this 
perpendicular  to  the  insertion  of  the  contracting 
muscles  will  show  a  large  displacing  component, 
acting  forward  and  upward.  But  it  must  be 
kept  in  mind  that  the  annular  ligament,  under 
which  the  tendons  of  the  muscles  in  question 
run,  modifies  this  theoretical  conclusion.  It  will 
appear  that  the  myodynamic  angle  made  by 
any  one  of  these  muscles  will  always  be  acute. 
And  hence,  the  displacing  component  woidd  act 
backward  and  downward,  and  would  be  repre- 
sented by  a  line  draw7i  from  the  insertion  of  the 
contracting  muscles  in  the  long  axis  of  the  foot 
till  it  meets  a  perpendicidar  drawn  from  the 
annular  ligament,  where  it  passes  over  the  tendons, 

loi.  (a.)   Surgeons  have  found  the  foot  dislo- 
cated backward  more  frequently  than  forward. 


THE    PRINCIPLES    OF    iMYODYNAMICS.  Io5 

The  dynamic  relations  of  the  muscles  that  span 
the  ankle-joint  and  move  the  foot  accord  with 
the  facts  of  observation.  The  backward  dis- 
placing components  preponderate  over  the  for- 
ward displacing  components. 

(b.)  The  foot  is  dislocated  outward  more 
frequently  than  inward.  The  following  dynamic 
relations  will  explain  this  fact  :  The  long  axis 
of  the  leg  at  the  tibio-fibular  arch  meets  the 
conjugate  axis  of  the  foot  at  a  considerable 
angle,  going  down  to  the  supporting  surface  on 
which  the  foot  rests,  near  the  inner  side  of  the 
foot,  the  conjugate  axis  of  the  foot  deviating 
outward  from  above  downward.  On  account  of 
this  outward  obliquity  of  the  foot,  when  the  foot 
forcibly  meets  a  resisting  surface,  the  force  of 
reaction  will  be  resolved  so  as  to  afford  a  con- 
siderable displacing  component  acting  outward. 
The  deltoid  ligament  is  apt  to  be  ruptured,  or 
the  internal  malleolus  may  be  broken  off,  and 
the  fibula  may  be  broken  above  the  ankle-joint. 

I02.  (A.)  Fig.  22  represents  the  tibia,  fibula, 
astragalus,  and  os  calcis,  as  seen  from  behind — 
one-half  natural  size.     The  line  a  j  b  is  in  the 


I06  THE    PRLNXIPLES    OF    MVODVXAMICS. 


THE    PRINCIPLES    OF    MYODYXAMICS.  lOj 

long  axis  of  the  tibia,  and  the  Hne  d  j  c  is  in  the 
conjugate  diameter  of  the  tarsus  :  These  lines 
meet  at  the  angle  b  j  d  The  line  b  j  represents 
the  retentive  pressure  of  the  tibio-fibular  arch 
on  the  astragalus,  and  the  line  b  d  represents 
the  inward  displacing  force  due  to  the  obliquity 
of  the  tarsus  :  because  the  rio-ht  anufled  trianHes 
d  j  b  and  a  j  c  are  similar — D  :  R  :  :  D,  :  R.  I 
have  treated  a  dislocation  of  the  foot,  compli- 
cated with  Potts'  fracture,  in  which  the  sole 
cause  was  the  displacing  component  marked  D  in 
the  Fig.,  a  heavy  weight  having  fallen  on  the  top 
of  the  knee,  when  the  foot  rested  on  the  ground 
and  the  leg  was  perpendicular  to  the  foot  :  The 
force  applied  in  this  case  must  have  been  very 
great. 

1 02.  (B.)  Fig.  23  represents  an  inside  view 
of  the  bones  of  the  foot  with  the  lower  end  of 
the  tibia  :  d  A  B  is  the  plantar  arch,  and  P-F 
is  the  plantar  fascia.  The  astragalus  A  is  the 
key-bone  of  the  arch,  having  the  weight  nearer 
the  posterior  than  the  anterior  pillar.  When 
the  heel  is  raised  the  weight  of  the  body  rests 
on   the  anterior  pillar,  and  when  the  toes   are 


I08  THE    PRINCIPLES    OF    MYODYNAMICS. 

raised  the  weight  of  the  body  rests  on  the 
posterior  pillar.  The  plantar  fascia  is  very 
important  in  the  construction  and  mechanism  of 
the  foot ;  so  rriuch  so,  when  it  is  too  lonor  or  too 
short,  the  function  of  the  foot  is  more  or  less  dis- 
turbed.— The  sub-astragoloid  joint  has  already 
been  noticed.  The  medio-tarsal  joint  is  where 
the  OS  calcis  and  astrao^alus  meet  the  cuboid  and 
scaphoid  bones,  and  is  a  ginglymus,  having  con- 
siderable flexion  and  extension.  The  medio- 
pedal  joint  is  where  the  os  calcis,  the  cuboid 
bone,  and  the  fourth  metatarsal  bone  meet  the 
astragalus,  the  scaphoid,  and  external  cruciform 
bones,  and  the  third  metatarsal  bone,  and  runs 
longitudinally  through  the  foot.  The  joint  be- 
tween the  head  of  the  astragalus  and  the 
scaphoid  bone  resembles  a  ball-and-socket  joint, 
and  facilitates  inversion  of  the  foot,  as  the  os 
calcis  glides  inward  under  the  astragalus,  the 
latter  bone  only  moving  backward  and  forward 
as  the  foot  is  flexed  and  extended. — The  foot  is 
admirable  for  its  strength  and  adaptation,  but  its 
usefulness  is  often  diminished  by  neglect  and 
ignorance. 


THE    PRINCIPLES    OF    MVODVXAMICS.  1 09 


no  THE    PRINCIPLES    OF    MVODVXAMICS. 


THE  LEG  AND  THE  KNEE-JOINT. 

103.  In  regard  to  the  knee-joint,  note  the 
following  points  : 

(i.)  The  articular  surface  of  the  tibia  for  the 
internal  condyle  of  the  femur  is  concave  and 
fitted  for  rotation. 

(2.)  The  articular  surface  of  the  tibia  for  the 
external  condyle  of  the  femur  is  convex  from 
before  backward  and  concave  from  side  to  side, 
and  fitted  for  a  o^lidinor  motion. 

(3.)  The  internal  condyle  is  larger  and  pro- 
jects downward  further  than  the  external  con- 
dyle. Hence^  the  femur  meets  the  tibia  so  as 
to  make  an  obtuse  anorle  on  the  outside  of  the 
limb. 

(4.)  The  oblique  fasciculus  of  the  posterior 
ligament  of  the  knee-joint  runs  from  the  pos- 
terior part  of  the  external  condyle  inward  and 
downward  to  the  inner  aspect  of  the  tibia — and 
limits  the  oict-rotation  of  the  leg. 

(5.)  The  anterior  crucial  ligament  runs  from 


THE    PRINCIPLES    OF    .MYODVXAMICS.  I  1  1 

the  inner  and  front  side  of  the  spine  of  the  tibia 
upward,  backward,  and  outward  to  the  inner 
and  back  part  of  the  external  femoral  condyle — 
and  li7nits  the  in-rotatio7i  of  the  leg,  and  prevents 
displacement  of  the  tibia  backward. 

(6.)  The  posterior  crucial  lig-ament  runs  from 
the  depression  behind  the  spine  of  the  tibia 
upward,  forward,  and  inward  to  the  outer  and 
fore  part  of  the  internal  femoral  condyle — and 
limits  ont-rotatio7i  of  the  leg,  and  prevents  dis- 
placemcrd  of  the  tibia  forivard, 

(7.)  The  leg  has  flexion,  extension,  and  rota- 
tion : — The  flexion  and  extension  of  the  leor 
takes  place  through  an  arc  somewhat  less  than 
two  ricrht  an  cries.  The  le^r  can  be  rotated 
throuo^h  an  arc  of  about  fifteen  decrrees.  The 
rotation  of  the  leg  takes  place  around  the  center 
of  the  tibial  socket  for  the  internal  condyle  of 
the  femur — the  external  socket  of  the  tibia  glid- 
ing backward  and  forward  under  the  external 
condyle  of  the  femur.  This  motion  can  be 
clearly  shown  when  the  leg  meets  the  thigh  at 
a  right  angle. 

104.    The   muscles   that  span    the   knee-joint 


1  I  2  THE    PRINCIPLES    OF    MYODYNAMICS. 

may  be  grouped,  according-  to  their  function,  as 
follows  : — 

(i.)  The  extensors  of  the  leg  are  the  muscles 
of  the  quadriceps  extensor. 

(2.)  T\\^  flexors  of  the  leg  are  the  semi-tendin- 
osus,  the  semi-membranosus,  the  biceps  cruris, 
the  popliteus,  the  gracilis,  the  sartorius,  and  the 
gastrocnemius. 

(3.)  The  oitt-rotators  of  the  leg  are  the  biceps 
cruris  and  the  sartorius. 

(4.)  The  ill-rotators  of  the  leg  are  the  semi- 
tendinosus,  the  semi-membranosus,  the  popli- 
teus, the  gracilis,  and  the  sartorius. 

io5.  (i.)  The  rectus  femoris,  the  sartorius, 
the  semi-tendinosus,  the  semi-membranosus,  the 
gracilis,  and  part  of  the  biceps  cruris  span  both 
the  knee-joint  and  the  hip-joint. 

(2.)  The  vastus  internus,  the  vastus  externus, 
the  popliteus,  and  part  of  the  biceps  cruris  span 
the  knee-joint. 

(3.)  The  gastrocnemius  spans  both  the  knee- 
joint  and  the  ankle-joint. 

106.  The  muscles  that  span  the  knee-joint 
may  make  the  femur  the  fixed   bone  and  the 


THE    PRINCIPLES    OF    MYODYNAMICS. 


113 


114  THE    PRINCIPLES    OF    MYODYNAMICS. 

tibia  the  movable  bone — or  the  tibia  the  fixed 
bone  and  the  femur  the  movable  bone.  The 
muscles  that  span  both  the  knee-joint  and  the 
hip-joint  may  make  the  femur  the  fixed  bone 
and  the  tibia  and  the  pelvis  the  movable  bones. 
107.  See  Fig.  24  :  Pa  is  the  patella  ;  q-e  is 
the  quadriceps  extensor  ;  Fe  is  the  femur  ;  and 
T  is  the  tibia  :  Draw  the  line  W'P-D  F  hav- 
ing a  right  angle  at  P  and  D  :  P  is  at  the 
insertion  of  the  tendo-patellae,  and  F  is  where 
the  tibia  and  femur  come  in  contact.  The  dis- 
tance P  D  is  about  two  inches,  and  the  length  of 
the  tibia  is  about  fourteen  inches  :  and  W  D  P 
is  a  bent  lever  of  the  first  order,  whose  fulcrum 
is  D  F.  Let  the  low^er  end  of  the  femur  rest  on 
a  resisting  surface,  and  let  a  weight  of  fifty 
pounds  be  placed  on  the  lower  end  of  the  tibia 
— always  acting  in  the  direction  P  D  of  the 
power-arm  of  the  lever.  By  the  principles  of 
the  lever  : 

2  X  P  11=  14  X  5o  .         (i.) 

Hence,         P  =:  360      .         .         .         (2.) 
But,  F  =  P  -f  W  =  400    .         (3.) 

Therefore,  the  pull  of  the  quadriceps  equals 


THE    PRINCIPLES    OF    MYODYXAMICS.  I  I :) 

35o  pounds,  and  the  pressure  on  the  joint- 
surfaces  equals  400  pounds.  It  may  be  re- 
marked, that  the  tendency  of  the  weight  is  put 
on  the  joint  surfaces  by  means  of  the  articular 
ligaments,  and  is  thus  added  to  the  retentive 
action  of  the  quadriceps  : — The  static  work  of 
the  bone  is  greater  than  the  kiiietic  work  of  the 
muscle. 

108.  See  Fig.  2  5  :  The  description  is  the 
same  as  before — except  that  the  tibia  and  the 
femur  meet  at  a  right  angle.  The  condyles  of 
the  femur  meet  the  base  of  the  tibia  further  back 
than  in  the  previous  instance.  The  distance 
P  D  is  somewhat  more  than  two  inches — but  let 
us  say  tw^o  inches.  The  distance  D  C  is  about 
four  inches.  The  component  triangle  is  P  D  C. 
The  rectangular  components  are  to  each  other 
as  2:4,  and,  since  the  weight  is  fifty  pounds, 
the  component  D  C  equals  360  pounds,  and  the 
component  PD  equals  176  pounds.  These 
two  components  will  be  resolved  from  the  force 
of  the  quadriceps — which  will  equal  391  pounds. 
But  D  C  is  a  retentive  as  well  as  a  movine  com- 
ponent ;    while  P  D  is  a  displacing  component, 


I  1 6  THE    PRINCIPLES    OF    MYODYNAMICS. 


THE    PRIXCTPLES    OF    MVODYXAMICS.  II7 

and  is  resisted  by  the  anterior  crucial  ligament, 
and  thus  has  its  force  brought  on  the  joint- 
surfaces,  so  that  the  two  rectangular  components 
hold  the  fixed  and  movable  bones  together  with 
a  force  equal  391  pounds. — Add  to  this  the 
weight  of  5o  pounds,  and  the  mutual  pressure 
of  the  joint-surfaces  of  the  tibia  and  femur 
equals  44 1  pounds. 

109.  See  Fig.  26  :  The  description  is  the 
same  as  in  the  first  instance — except  that  the 
femur  and  the  tibia  meet  at  an  acute  anele. 
The  bent  lever  is  P  d  W,  having  a  fulcrum  Fd. 
The  myodynamic  angle  is  C  P  D — the  conjugate 
axis  of  the  base  of  the  tibia  prolonged  being  one 
side  of  the  component  triangle.  In  this  case  the 
rectangular  components  are  equal.  The  moving 
component  DC  equals  35o  pounds  as  before, 
when  the  weight  equals  5o  pounds,  and  the  dis- 
placing component  P  D  equals  350  pounds. 
The  force  of  the  quadriceps,  from  which  the  two 
equal  rectangular  components  are  resolved,  is 
therefore  equal  494  pounds — and  the  pressure 
of  the  condyles  of  the  femur  on  the  base  of  the 
tibia  would  be  644  pounds. 


Il8  THE    PRIiNXIPLES    OF    MVODYNAMICS. 

I  lo.  If  now  the  weight  of  the  body  be  put  on 
the  tibia  as  a  lever,  the  tibia  will  have  the  same 
dynamic  relation  to  the  femur  as  the  foot  has  to 
the  bones  of  the  leg,  and  the  tibia  will  be  a  lever 
of  the  second  order. 

(i.)  In  case  the  body  is  erect,  and  on  the 
supposition  that  there  is  no  action  of  the  muscles 
that  span  the  knee-joint,  the  weight  of  the  body 
minus  the  weight  of  the  legs  will  rest  on  the 
bases  of  the  tibiae  ;  and  the  muscular  force  re- 
quired to  maintain  the  body  erect  at  the  knees 
must  be  added  to  this  weight. 

(2.)  Let  the  body  weigh  i5o  pounds,  and  let 
it  rest  on  the  tibial  levers  meeting  the  femora 
at  right  angles  :  The  pressure  on  the  joint- 
surfaces  would  be  over  1 200  pounds  :  on  two 
condyles  the  pressure  would  be  600  pounds  : 
and  on  one  condyle  the  pressure  would  be  300 
pounds. — But,  if  the  weight  of  the  body  rested 
on  one  leg,  the  pressure  on  two  condyles  would 
be  1200  pounds  ;  and  the  pressure  on  one  con- 
dyle would  be  600  pounds. 

(3.)  It  appears  that,  as  the  myodynamic  angle 
C  P  D  decreases  or  becomes  less  than  a  right 


THE    PRINXIPLES    OF    >[YODYNAMICS.  II9 


120  THE    PRINCIPLES    OF    MVODVXAMICS. 

angle,  the  force  of  the  quadriceps  must  increase 
as  well  as  the  mutual  pressure  of  the  ends  of  the 
bones  ;  and  these  facts  take  place  in  the  order 
indicated  during  flexion  of  the  leg.  The  patella 
auo^ments  the  mvodvnamic  anole  :  Hence  the 
patella  makes  the  quadriceps  act  to  greater 
advantage,  and  relatively  diminishes  the  mutual 
pressure  of  the  ends  of  the  bones. 

111.  These  conclusions  may  be  demonstrated 
substantially  by  the  following  experimental 
means  :  Take  the  bones  of  the  lower  limb,  as 
they  are  articulated  in  a  skeleton,  and  fasten  a 
dynamometer  to  the  upper  end  of  the  patella  by 
means  of  a  strincr  ;  when  the  leo^  is  extended, 
the  dynamometer  will  have  its  index  at  a  certain 
figure,  and  as  the  leg  is  constantly  flexed  by 
means  of  its  own  weight,  the  index  of  the  dy- 
namometer will  as  constantly  point  to  a  greater 
fiorure. 

112.  When  the  leg  is  completely  extended, 
the  semi-membranosus  has  a  larger  moving  com- 
ponent than  the  semi-tendinosus,  because  it  acts 
on  a  longer  power-arm  :  See  P'  s-m,  Fig.  24. 
But  when  the  tibia  meets  the  femur  at  a  right 


THE    PRINCIPLES    OF    MVODYXAMICS.  I  2  I 

angle,  the  semi-tendinosus  has  a  greater  moving 
component  than  the  semi -membranosus,  because 
it  acts  on  a  longer  power-arm  :  See  Fig.  2  5. 
Hence,  during  the  first  part  of  flexion  of  the 
leg,  the  semi-membranosus  is  the  more  powerful 
agent  ;  and  during  the  second  part  of  flexion  of 
the  leg,  the  semi-tendinosus  is  the  more  power- 
ful aofent. — In  these  instances  the  tibia  becomes 
a  lever  of  the  third  order.  When  the  leo^  is 
extended,  the  moving  components  are  retentive : 
but  when  the  leg  is  semiflexed,  the  moving 
components  are  displacing. 

113.  a.  In  a  given  case  the  following  meas- 
urements were  made  : — 

(i.)  From  the  center  of  rotation  of  the  leg 
about  the  internal  condyle  (which  is  the  fulcrum) 
outward  to  a  point  in  a  line  drawn  directly 
upward  from  the  insertion  of  the  biceps  cruris, 
the  distance  was  about  two  and  one-half  inches  : 
This  would  constitute  the  power-arm  of  a  lever, 
whose  power  is  the  biceps  cruris. 

(2.)  From  the  same  center  of  rotation  inward 
to  a  point  in  a  line  drawn  directly  upward  from 


122  THE    TRINXIPLES    OF    MVODVXAMICS. 

the  inner  part  of  the  semi-membranosus  the 
distance  was  about  one-half  inch. 

(3.)  Hence,  other  things  being  equal,  the 
outrotating  power  of  the  biceps  cruris  is  five 
times  as  great  as  the  in-rotating  power  of  the 
semi-membranosus. 

(4.)  x\nd,  as  the  insertion  of  the  semi-tend- 
inosus  is  relatively  nearer  the  center  of  rotation 
of  the  tibia,  the  semi-membranosus  has  a  greater 
in-rotating  power  than  the  semi-tendinosus. 

b.  In  the  same  case  the  following  measure- 
ments were  also  made  : — 

(i.)  The  distance  from  the  knee-joint  to  the 
insertion  of  the  biceps  cruris  was  about  one-half 
inch. 

(2.)  The  distance  from  the  knee-joint  to  the 
insertion  of  the  semi-tendinosus  was  about  two 
and  one-half  inches. 

(3.)  Hence,  other  things  being  equal,  the 
semi-tendinosus  has  five  times  the  fiexino^ 
power  of  the  biceps  cruris. 

114.  In  fact,  the  special  rotating  antagonist 
to    the    biceps    cruris    is    the    popliteus  : — The 


THE    PRIN'CIPLES    OF    MYODVXAMICS.  I  23 

popliteus  pulls  on  the  external  condyle  and  in- 
rotates  the  leo^. 

ii5.  In  those  cases  of  triple  displacement  of 
the  leg,  on  account  of  disease  :  (i.)  The  out- 
rotation  is  due  to  the  biceps  cruris  ;  (2.)  The 
flexion  is  due  to  the  semi-membranosus  and  the 
semi-tendinosus  ;  (3.)  The  sub-luxation  is  due 
to  the  semi-membranosus,  the  biceps  cruris,  and 
the  semi-tendinosus. 

116.  Some  general  conclusions  may  now  be 
made  in  regard  to  the  stability  of  the  knee-joint, 
when  the  leg  is  in  different  positions  : — 

(i.)  The  condition  of  maximum  stability  of 
the  knee-joint  is  when  the  leg  is  completely 
extended,  for  then  the  retentive  components  of 
all  the  muscles  that  span  the  knee-joint  have  a 
maximum  magnitude.  This  condition  of  sta- 
bility of  the  knee-joint  gradually  diminishes 
until  the  tibia  nearly  meets  the  femur  at  right 
ano^les. 

(2.)  The  condition  of  minimum  stability  of 
the  knee-joint  is  in  the  vicinity  of  semi-flexion 
of  the  leg.  The  displacing  components  of  the 
muscles  that  span  the  knee-joint  are  very  great 


124         THE    PRINCIPLES    OF    MYODYNAMICS. 

in  the  vicinity  of  complete  flexion — except  those 
of  the  gastrocnemius  and  the  popHteus  : — And 
when  the  leg  is  meeting  the  thigh  at  an  acute 
angle,  the  popliteus  and  the  gastrocnemius  will 
antagonize  the  ham-string  muscles  and  thus 
contribute  to  the  stability  of  the  knee-joint : 
Besides,  the  tibia,  if  displaced  backward,  would 
soon  come  against  the  femur,  and  so  a  complete 
dislocation  would  be  prevented. 


THE    FEMUR    AND    THE    HIP-JOINT. 

117.  In  regard  to  the  hip-joint,  the  following 
points  are  to  be  noted  :  (i.)  The  hip-joint  is  a 
ball-and  socket  joint ;  (2.)  The  socket  is  deep  ; 
(3.)  The  head  of  the  femur  is  two-thirds  of  a 
spheroid  ;  (4.)  The  average  length  of  the  femoral 
head  is  about  one  and  two-tenths  inches ;  (5.) 
The  averao^e  leno^th  of  the  femoral  neck  is  about 
one  and  four-tenths  inches  ;  (6.)  The  average 
lateral  diameter  of  the  trochanters  at  the  junc- 
tion of  the  femoral  neck  is  about  one  and  five- 
tenths  inches  ;   (7.)  The  average  distance  from 


TIIF    PRINXIPLES    OF    MVODVXAMICS.  120 

the  apex  of  the  femoral  head  in  the  long  axis  of 
the  femoral  neck  to  the  outer  surface  of  the 
great  trochanter  is  about  four  inches';  (8.)  The 
neck  of  the  femur  generally  meets  the  shaft  at 
an  obtuse  angle  from  above  downward;  the 
neck  may,  however,  be  depressed  till  it  meets 
the  shaft  at  a  riorht  anijle — or  even"  an  acute 
angle — especially  in  old  age  ;  (9.)  The  length 
of  the  adult  femur  is  about  seventeen  inches. 

118.  The  two  following  facts  are  worthy  of 
record — namely, 

( I .)  One  of  the  lower  limbs  is  generally  about 
one-fourth  of  an  inch  longer^ than  the  other, — 
constituting  normal  a -symmetry  of  the  lower 
limbs. 

(2.)  When  the  upper  point  of  meas^urement  is 
the  superior  anterior  spine  of  the  ilium, — ab- 
dMction  shortens  the  measurement  of  the  lower 
limb, — adduction  lengthens  the  measurement  of 
the  lower  limb. 

(3.)  Hence,  other  things  being  -equal,  that 
lower  limb  will  measure  longer  than  its  fellow, 
Avhich  is  on  the  side  of  the-  pelvis  that  is  tilted 
upward. 


126  THE    PRIXC'TPI.ES    OF    MVt)nVXAMICS. 

119.  The  motions  of  the  femur  are — adduc- 
tion, flexion,  abduction,  extension,  in-rotation, 
oiit-rotation.  and  circumduction  : — 

(i.)  The  addiictors  of  the  femur  are — The 
proas  magnus,  the  iUacus,  the  pectineus,  the 
graciHs,  the  adductor  brevis.  the  adductor 
longus.  the  adductor  magnus.  the  quadratus 
femoris,  the  obturator  extensus,  the  semi-mem - 
branosus,  the  semi-tendinosus,  the  biceps  cruris, 
and  the  inferior  part  of  the  ghiteus  maximus. 

(ii.)  The  flexors  of  the  femur  are — Fhe  an- 
terior part  of  the  gluteus  minimus,  the  psoas 
magnus,  the  ihacus,  the  pectineus,  the  graciHs, 
the  adductor  brevis,  the  adductor  longus,  the 
sartorius,  the  tensor  vaginae  femoris.  the  rectus 
femoris,  and  the  obturator  externus. 

(iii.)  The  abductors  of  the  femur  are — The 
gluteus  minimus,  the  o-hiteus  medius,  the  o^lu- 
teus  maximus,  the  tensor  vaginae  femoris,  the 
sartorius.  the  pyriformis,  and  the  rectus  femoris. 

(iv.)  The  extensors  of  the  femur  are — The 
gluteus  maximus,  the  biceps  cruris,  the  semi- 
membranosus, the  semi-tendinosus,  and  the 
adductor  maenus. 


THE    PRINXIPLES    OF    MVODVXAMICS.  1 27 

(v.)  The  ill-rotators  of  the  femur  are — (i.) 
The  ghiteus  minimus  ;  (2.)  The  gluteus  medius ; 
(3)  the  tensor  vaginae  femoris  ;  (4.)  The  vastus 
externus  ;  (5.)  The  vastus  internus  ;  (6.)  The 
crureus  ;  (7.)  The  rectus  femoris  ;  (8.)  The 
gracihs  ;  (9.)  The  semi-membranosus  ;  (10.) 
The  semi-tendinosus  ;  (11.)  The  ihacus  ;  (12.) 
The  psoas  magnus  ;  (13.)  The  pectineus  ;  (14.) 
The  adductors. 

(vi.)  The  out-rotators  of  the  femur  are— (i.) 
The  gluteus  minimus  ;  (2.)  The  gluteus  medius ; 
(3.)  The  gluteus  maximus  ;  (4.)  The  pyriformis ; 
(5.)  The  obturator  internus  and  the  gemelli ;  (6.) 
The  obturator  externus  ;  (7.)  The  quadratus 
femoris  ;  (8.)  The  biceps  cruris  ;  (9.)  The  sar- 
torius  ;  (10.)  The  abductors;  (11.)  The  pec- 
tineus ;  (12.)  The  iliacus  ;  (13.)  The  psoas 
magnus. 

(vii.)  The  circumditctors  of  the  femur  are  all 
the  muscles  that  span  the  hip-joint. 

120.  It  ought  to  be  especially  noted  that  the 
origin  of  the  obturator  externus  is  considerably 
below  its  insertion,  so  that  this  muscle  tends — 
(i.)to  prevent  the  shaft  of  the  femur  from  going 


128  THE    PRINXIPLES    OF    MVDDYXAMICS. 

upward  when  the  femoral  neck  is  broken  ;  (2.) 
to  prevent  upward  dislocation  of  the  femur  : — 
Also  the  obturator  internus  will  have  similar, 
thoueh  less  marked,  functions. 

12  1.  The  biceps  cruris,  the  semi-membran- 
osus,  the  semi-tendinosus,  the  cr^'acilis,  the 
sartorius,  and  the  rectus  femoris  span  the  hip- 
joint  and  the  knee-joint.  The  gluteus  maximus, 
the  elnteus  minimus,  the  crhiteus  medius,  the 
tensor  vaginae  femoris,  the  psoas  magnus,  the 
iliacus,  the  pectineus,  the  adductor  magnus,  the 
adductor  longus,  the  adductor  brevis,  the  obtur- 
ator externus,  the  triceps  rotator,  and  the  pyri- 
formus  span  only  the  hip-joint. —  The  sartorius 
flexes,  out -rotates,  and  abducts  the  thigh  :  and 
flexes  the  leg. 

122.  The  shaft  of  the  femur  does  not  rotate 
directly  on  its  own  axis  ;  it  rotates  indirectly,  as 
it  moves  forward  and  backward  under  the  action 
of  rotator  muscles.  When  the  femoral  neck  is 
broken,  the  shaft  of  the  femur  can  rotate  di- 
rectly on  its  own  axis. 

123.  When  the  femur  is  flexed  and  extended, 
the  femoral  neck  will  rotate  nearlv  on  its  own 


THE    PRINXIPI.es    OF    MYODYXAMICS.  1 29 

axis.  The  head  and  neck  of  the  femur  may  be 
looked  upon  as  a  lever.  The  entire  femur  is  a 
befit  lever.  The  acetabiditm  is  the  fulcrum.  When 
the  femur  is  the  movable  bone,  the  components 
of  the  muscles  that  span  the  hip -joint  may  be 
determined  on  the  principle  of  a  lever  of  the 
third  order.  When  the  body  rests  on  the  head 
of  the  femur,  and  is  moved  by  the  muscles  that 
span  the  hip-joint,  these  muscles  act  on  the 
pelvis  as  the  power  of  a  lever  of  the  first  order. 
The  femur  may  be  rotated  about  forty-five 
degrees.  The  femur  may  be  adducted  from  a 
line  parallel  with  the  long  axis  of  the  body 
about  thirty  degrees.  The  femur  may  be  ab- 
ducted from  a  line  parallel  with  the  long  axis  of 
the  body  about  thirty  degrees.  The  lateral 
motion  of  the  lower  limb  will  therefore  be  not 
far  from  sixty  degrees.  The  femur  may  be 
flexed  till  the  thio^h  comes  in  contact  with  the 
surface  of  the  abdomen.  The  femur  can  be 
extended  so  that  the  condyles  will  be  somewhat 
back  of  the  long  axis  of  the  body  prolonged. 
124.  It  requires  a  few  words  in  regard  to  the 


130  THE    PRTNXIPLES    OF    MYODVXAMTCS. 

rotating  function  of   tJic  flexors  of  the  thigh — 
that  is,  the  psoas  magnus  and  the  ihaciis  : — ' 

When  the  thigh  is  parallel  with  the  long  axis 
of  the  body,  and  when  the  femoral  neck  is  of 
average  lenoth,  and  when  the  trochanter  minor 
is  of  moderate  length,  the  insertion  of  the 
flexors  of  the  thicrh,  during-  in-rotation  of  the 
femur,  will  move  forward  and  inward  faster  than 
backward  and  outward,  inasmuch  as  the  femoral 
neck  makes  a  longer  radius  than  the  semi -diam- 
eter of  the  shaft  of  the  femur  plus  the  height  of 
the  trochanter  minor  :  The  sanu-  may  bt^  said 
when  thti  thigli  is  abducted.  I  ndt*r  the  condi 
tions  named,  the  psoas  magnus  and  the  iliacus 
are  in -rotators  of  the  femur.  But  when  the 
thi2"h  is  adducted,  and  when  the  trochanter 
minor  is  largely  developed,  so  as  to  make  a 
lever  long  enough,  the  psoas  magnus  and  the 
iliacus  are  out-rotators  of  the  femur.  These 
luiiscles  are  flexors  of  the  tJiigh. 

12  5.  The  triceps  rotator  of  the  femur  is  made 
up  of  the  gemelli  and  the  ol)turator  internus. 
In  oeneral.  when  the  lono-  axis  of  the  femur  and 


THE    PRINCIPLES    OF    MVODYNAMICS.  131 

the  long  axis  of  the  body  are  parallel,  the  down- 
ward obliquity  of  the  pull  of  the  triceps  rotator 
is  about  the  same  as  the  upward  obliquity  of  the 
femoral  neck  :  Hence,  the  downward  displacing 
component  of  the  triceps  rotator  will  be  about 
counteracted  by  the  upward  displacing  tendency 
on  account  of  the  obliquity  of  the  femoral  neck. 

126.  When  the  femur  is  completely  in-ro- 
tated, the  rotating  component  of  the  triceps 
rotator  will  be  small  ;  but  will  increase  in  size 
during  out-rotation  of  the  femur,  when  the 
myodynamic  angle  will  be  at  the  maximum. 
On  account  of  the  conformation  of  the  hip-joint, 
the  triceps  rotator  will  always  have  a  large  re- 
tentive and  a  small  displacing  component. 

127.  The  anterior  part  of  the  gluteus  mini- 
mus has  four  components  :  (i.)  A  moving 
component  ;  (2.)  A  retentive  component  ;  (3.) 
A  rotating  component  ;  and  (4.)  A  displacing 
component  : — In  this  case,  the  retentive  com- 
ponent is  always  large — on  account  of  the 
conformation  of  the  hip-joint. 

128.  Let  us  now  examine  the  dynamic  rela- 


132  THE    PRINCIPLES    OF    MYODYNAMICS. 

tions  of  the  triceps  rotator  and  the  inrotating 
part  of  the  gkiteiis  minimus  by  measuring  a 
given  case  : — 

(i.)  From  the  insertion  of  the  gluteus  mini- 
mus to  the  insertion  of  the  triceps  rotator,  the 
distance  was  one  inch. 

(2.)  From  the  insertion  of  the  triceps  rotator 
to  the  head  of  the  femur,  the  distance  was  tw^o 
and  one-half  inches. 

(3.)  And  from  the  insertion  of  the  gluteus 
minimus  to  the  head  of  the  femur  the  distance 
was  three  and  one  half  inches. 

129.  Let  the  rotating  component  of  the  tri- 
ceps rotator  be  the  power,  the  rotating  compon- 
ent of  the  o["luteus  minimus  be  the  weio^ht,  and 
the  pressure  on  the  head  of  the  femur  be  the 
fulcrum  : — The  lever  will  be  of  the  third  order  : 
Let  the  fulcrum  be  fifty  pounds. 

Then:      P  =  W  +  5o       .          .          .  (i.) 

And:       W  =|x  F  =  125        .          .  (2.) 

Hence     P  =  175      ....  (3.) 

130.  These  figures  show  : — 

(i.)  The  rotating  component   of  the   gluteus 


THE    PRINXIPLES    OF    MYODYXAMICS.  1 33 

minimus  has  a  more  advantageous  position  and 
dynamic  relation  than  the  rotating  component 
of  the  triceps  rotator. 

(2.)  The  rotating  component  of  the  triceps 
rotator  equals  the  rotating  component  of  the 
gluteus  minimus  plus  the  pressure  on  the  fe- 
moral head  in  the  direction  of  the  motion  of  the 
rotating  component  of  the  triceps  rotator. 

(3.)  But  when  the  articular  ligaments  and  the 
conformation  of  the  joint  cause  the  combination 
of  the  muscular  components  under  considera- 
tion, the  pressure  on  the  head  of  the  femur  is 
equal  the  sum  of  the  contractile  energy  of  the 
two  muscles. 

131.  (a.)  To  a  certain  extent,  a  long  femoral 
neck  has  favorable  dynamic  relations  : — 

(i.)  This  is  true  in  regard  to  the  lateral  strain 
caused  by  the  moving  components  of  the  con- 
tracting muscles. 

(2.)  This  is  not  true  in  regard  to  the  pressure 
caused  by  the  entire  force  of  the  contracting 
muscles. 

(b.)  The  weakest  part  of  the  femoral  neck  is 
at  its  junction  with  the  head  : — 


134  THE    PRINXIPLES    OF    MVODYNAMICS. 

(i.)  This  fact  agrees  with  the  need  of  having 
the  cervical  lever  of  orreater  strenorth  the  nearer 
we  get  to  the  insertion  of  the  muscles. 

(2.)  In  this  place,  it  may  be  remarked  that 
the  femoral  neck  of  the  female  is  oftener  broken 
than  the  femoral  neck  of  the  male,  and  that  the 
femoral  neck  of  the  female  is  not  unfrequently 
broken  near  the  femoral  head. 

132.  The  gluteus  medius  is  favorably  located 
to  apply  great  force  to  the  femoral  neck  : — 

See  Fig.  27  :  Fe  is  the  femur,  which  is  a  bent 
lever :  g  m  is  the  gluteus  medius,  whose  origin 
and  insertion  are  o  and  i  ;  H  is  the  femoral 
head  ;  c  c  are  the  femoral  condyles  ;  F  is  the 
fulcrum  ;  P  is  the  power ;  and  W  is  the  weight. 
The  femoral  neck  is  a  lever  of  the  third  order 
under  the  action  of  the  gluteus  medius  : — Pro- 
lone  the  loner  axis  of  the  femoral  neck — inward, 
if  necessary,  till  it  meets  at  d  a  perpendicular 
drawn  from  the  origin  of  the  contracting  muscle 
— outward,  till  it  meets  a  perpendicular  drawn 
from  the  lower  end  of  the  femur  ;  and  prolong 
the  lono-  axis  of  the  contractino^  muscle  till  it 
meets  at  a.  the  prolonged  axis  of  the  femoral 


THE    PRINXin.ES    OF    MVODVX.vArTCS. 


1^0 


136  THE    PRTNXIPLES    OF    MVODYNAMICS. 

neck  : — The  rectangular  components  of  the 
contractinor  muscle  will  be  a  d  and  d  o  ;  and  a  d 
will  be  a  retentive  component  and  d  o  will  be  a 
moving  component.  In  a  given  case,  as  ap- 
proximately measured,  the  retentive  component 
ad  was  9,  and  the  moving  component  do  was 
3  :  Hence,  the  force  of  gluteus  medius  was 
9.5  nearly. 

133.  Now  let  the  weight  of  the  lower  limb 
pull  directly  downward  in  the  direction  i  P  W  : 
draw  i  e  perpendicular  to  P  F  :  Then  will  i  e 
and  e  p  be  the  rectangular  components  of  the 
force  of  the  weight  of  the  lower  limb.  But  i  e 
is  a  resisting  component  and  e  p  is  a  displacing 
component  ;  and  e  p  will  tend  to  counteract  the 
retentive  component  a  d  ;  and  i  e  will  counteract 
the  moving  component  d  o  :  The  force  of  i  e 
will  equal  the  force  of  d  o  ;  but  the  force  of  e  p 
will  be  less  than  the  force  of  a  d.  Draw  o  H 
parallel  to  i  p  :  1  hen  will  d  H  represent  the 
force  of  e  p  : — This  will  reduce  i  e  and  o  d,  and 
p  e  and  H  d  to  the  same  denomination  : — And 
then  will  a  H  represent  the  retentive  compon- 
ent of  the  power  and  the  weight  acting  on  the 


THE    PRI^XTPLES    OF    MYODYNAMICS.  1 37 

lever  P  F.  And  by  careful  comparative  meas- 
urements and  estimates  of  the  lines  and  the 
forces  they  represent,  it  appears  that  the  head 
of  the  femur  is  pressed  into  the  acetabulum  with 
a  force  at  least  twice  as  orreat  as  the  weight  of 
the  lower  limb,  when  that  limb  is  freely  sus- 
pended as  above  indicated  under  the  action  of 
the  gluteus  medius. 

1 34.  When  the  long  axis  of  the  gluteus  me- 
dius is  more  vertical  and  runs  in  the  direction 
o'  i,  then  the  moving  component  will  be  greater 
and  the  retentive  component  will  be  smaller. 
Also  Vv^hen  the  femoral  neck  meets  the  femoral 
shaft  more  nearly  at  a  right  angle,  then,  too, 
the  moving  component  will  be  greater  and  the 
retentive  component  will  be  smaller.  And,  fur- 
thermore, when  the  pelvis  is  tilted  outward  so 
as  to  brine  the  origins  of  the  Mutei  medius  and 
minimus  more  directly  over  the  trochanter  ma- 
jor, any  force  acting  downward  in  the  long  axis 
of  the  femur  will  antagonize  the  retentive  com- 
ponents of  these  two  muscles,  and  reduce  the 
pressure  between  the  femoral  head  and  the 
acetabulum    to    a    minimum.       These    dynamic 


138  THE    PRIXCIPLES    OF    MYODYNAMICS- 

facts  have  important  bearings  in  the  treatment 
of  hip-joint  disease  and  fracture  of  the  femoral 
neck. 

135.  If  the  weight  acts  on  the  lower  end  of 
the  femur  in  the  direction  of  b  w — w  b  d  being 
a  right  angle — then  the  entire  weight  will  resist 
the  moving  component  of  the  muscle  :  And 
the  orreater  the  weio^ht,  the  crreater  the  mo  vino- 
component  of  the  muscle  ;  but  the  greater  the 
moving  component  of  the  muscle,  the  greater 
the  retentive  component  of  the  muscle  :  Hence 
the  greater  the  weight,  the  greater  the  pressure 
of  the  head  of  the  femur  on  the  surface  of  the 
acetabulum. 

138.  If  the  weight  act  on  the  lower  end  of 
the  femur  in  any  direction  s  w  beyond  the  per- 
pendicular w  b,  the  rectangular  components  will 
be  s  b  and  b  w :  s  b  will  be  a  retentive  compon- 
ent, augmenting  the  action  of  the  retentive 
component  of  the  muscle,  while  b  w  will  be  a 
resisting  component,  antagonizing  the  moving 
component  of  the  muscle  :  And  the  nearer  the 
weight  acts  at  right  angles  to  the  femoral  shaft, 
the  greater  will  be  the  retentive  components. 


THE    PRINCIPLES    OF    MVODYNAMICS.  1 39 

1 39.  The  psoas  magnus  acts  on  the  femur, 
makine  it  a  lever  of  the  third  order.  In  a  oriven 
case,  the  following  approximate  measurements 
were  made  :  (i.)  The  length  of  the  femur  was 
about  seventeen  inches,  that  is  the  weight-arm 
of  the  lever  :  (2.)  The  distance  from  the  tro- 
chanter minor  in  the  line  of  the  rectangular 
component  of  the  psoas  magnus  to  the  head  of 
the  femur  was  about  four  inches,  that  is  the 
power-arm  of  the  lever.  See  Fig.  20  :  T-m  is 
the  trochanter  minor,  and  E  is  the  ilio-pectineal 
eminence,  while  P  is  the  distance  between  these 
two  points  :  The  rectangular  components  are 
3  and  4,  while  the  resultant  is  5.  The  result- 
ant expresses  the  entire  force  of  the  psoas 
magnus.  Let  the  weight  applied  to  the  femoral 
condyles  be  twelve  pounds  :  The  formula  for 
the  femoral  lever  will  be— 

12  X  17  =::=:  5i  X  4  : — 
Then  the  moving  component  of  the  psoas  mag- 
nus would  be  fifty-one  pounds  :  Divide  this  by 
three  and  multiply  the  quotient  by  four  and  it 
gives  a  retentive  component  of  sixty-eight 
pounds  :      Multiply  the    same  quotient  (seven- 


140  THE    PRINXIPLES    OF    MVODVXAMICS. 

teen)*  by  five  and  it  gives  the  entire  force  of  the 
psoas  magnus,  which  equals  eighty-five  pounds. 
And  this  means  that  the  femoral  head  is  pressed 
against  the  acetabular  surface  by  the  psoas  mag- 
nus with  a  force  equal  eighty-five  pounds  : — 
Now,  suppose  the  weight  of  the  lower  limb  to 
be  twelve  pounds,  and  the  weight  of  the  body 
to  be  sixty  pounds  :  and  if  the  psoas  magnus 
lifts  the  weight  of  the  lower  limb  applied  to  the 
condyloid  end  of  the  femoral  lever,  it  will  press 
the  hip-joint  surfaces  together  with  a  force  of 
eighty-five  pounds  :  But  if  the  weight  of  the 
body  rests  on  one  limb,  it  will  press  the  hip- 
joint  surfaces  together  with  a  force  of  only  sixty 
pounds.  The  psoas  magnus  can  do,  under  the 
circumstances,  more  work  than  the  weight  of 
the  body. 

140.  Let  the  weight  on  the  condyloid  end  of 
the  femoral  lever  be  twenty-four  pounds  : — 
Then  the  force  of  the  proas  magnus  will  be  170 
pounds. — Let  the  entire  force  of  the  rest  of  the 
muscles  that  span  the  hip-joint  be  ten  times  as 
great  as  the  force  of  the  psoas  magnus,  then  the 


THE    TRINCIPLES    OF    MVODYNAMICS.  I4I 

pressure  of  the  surface  of  the  femoral  head  on 
the  acetabular  surface  will  equal   1,700  pounds. 

141.  Some  practical  observations  may  be 
made  in  this  connection  : — 

(i.)  The  myodynamic  facts,  as  above  ex- 
plained, forbid  us  to  treat  the  active  conditions 
of  hip-joint  disease  by  permitting  the  patient  to 
walk  about — because  walkinof  about  causes  the 
muscles  of  the  hip,  even  under  surgical  appli- 
ances, to  contract — and  we  now  know  what 
great  pressure  these  muscles  can  make  on  the 
surfaces  of  the  hip  joint. 

(2.)  If  we  put  the  patient  on  the  back,  and 
make  extension  of  the  lower  limb  on  the  af- 
fected side,  we  accomplish  two  important  results  : 
(a.)  The  side  of  the  pelvis  on  the  affected  side 
is  tilted  downward,  and  that  shortens  the  lower 
limb,  when  the  measurement  is  made  from  the 
pelvis. — (b.)  And  the  lower  limb  is  pulled  more 
in  a  line  with  the  long  axis  of  the  glutei  medius 
and  minimus  :  TJitis  dinihiisJiing  pressiire  be- 
tween the  hip -joint  surfaces  by  diminishing  the 
size  of  retentive  muscular  components. 


142  THE    PRINCIPLES    OF    MYODYNAIVIICS. 

142.  In  regard  to  the  stability  of  the  hip-joint 
two  important  statements  may  be  made  : — 

(i.)  The  conformation  of  the  hip-joint  is  such 
that,  under  all  ordinary  myodynamic  relations, 
the  muscles  that  span  this  joint  have  greater 
retentive  than  displacing  components  : — where- 
fore, the  hip-joint  is  in  a  condition  of  myody- 
namic stability. 

(2.)  If  the  femur  is  moved  in  any  direction, 
so  as  to  brinor  the  femoral  neck  against  the 
brim  of  the  acetabulum,  the  femur  will  become 
a  lever  of  the  first  order,  which  will  tend  to  lift 
the  femoral  head  out  of  its  socket  when  the 
powerful  retentive  components  of  the  hip-joint 
muscles,  will  be  turned  into  displacing  compon- 
ents, so  that  external  violence  and  muscular 
force  will  co-operate  to  dislocate  the  femur  : 
Under  the  circumstances,  the  hip-joint  is  in  a 
condition  of  dynamic  instability. — And  these 
facts  agree  with  the  practical  observations  of 
suro^eons. 

143.  The  femoral  shaft  has  an  arch  which 
bends  forward,  and  this  throws  the  lesser  troch- 


THE    PRINCIPLES    OF    MYODYXAMICS.  1 43 

anter  somewhat  backward.  Let  a  normal  femur 
be  laid  on  a  plane  surface  on  its  condyles,  and 
its  lesser  trochanter  and  the  femoral  head  will 
rise  some  distance  above  the  plane  surface  : 
This  brings  the  femoral  head  forward,  or  the 
internal  condyle  backward,  just  as  the  condyle 
or  the  head  is  looked  upon  as  the  fixed  point. 
In  another  place  I  have  shown  how  a  fracture 
may  derange  this  normal  twist  of  the  femur. 
Now,  if  the  leg  be  completely  extended,  and  a 
tape-line  fixed  over  the  femoral  head  and  over 
the  apex  of  the  tibio-fibular  arch,  it  will  run 
directly  over  the  long  axis  of  the  tibia,  and  also 
deviate  from  the  long  axis  of  the  femur  as  it 
runs  from  the  base  of  the  tibia  upward  to  the 
femoral  head.  And  the  meaninor  of  this  fact  is 
this — that,  as  the  weight  of  the  bod}'  rests  on 
the  femoral  head,  it  is,  as  a  force,  directly  over 
the  long  axis  of  the  tibia,  and  directly  over  the 
tibio-fibular  arch,  and  also  directly  over  the 
key-bone  of  the  plantar  arch.  I  have  verified 
these  facts  by  a  number  of  observations  and 
measurements. 

144.    I  have  also  made   some  measurements 


144  THE    PRINXIPLES    OF    MYODYXAMICS. 

showing  that  the  thigh  is  generally  equal  in 
length  to  the  leo;  and  vertical  diameter  of  the 

o  o 

foot  :  The  height  of  the  plantar  arch  and  the 
leneth  of  the  tibia  make  a  lever  of  about  the 
same  leneth  as  the  femur.  Hence,  when  the 
foot-and-lee  lever  and  the  femoral  lever  are 
antagonistic,  and  have  equal  lever-arms,  they 
are  in  a  condition  of  dynamic  equality,  or  per- 
haps it  might  be  more  correct  to  say  that  they 
are  in  equilibrium. 


THE    SKULL    AND    THE    CRAXTO-VERTEBRAL    JOINT. 

136.  The  condyles  of  the  skull  rest  on  the 
atlas,  and  the  atlas  rests  on  the  axis  ;  and  the 
odontoid  process  of  the  axis  is  where  the  body 
of  the  atlas  would  be — if  it  had  one.  The  skull 
has  the  following  motions — namely, 

(a.)  (i.)  Flexion  ;  (ii.)  extension ;  (iii.)  right 
rotation  ;  (iv.)  left  rotation  ;  (v.)  right  adduc- 
tion ;   (vi.)  left  adduction. 

(i.)  The^exo7's  of  the  skull  are  :  The  rectus 
capitis  anticus  minor,  the  rectus  capitis  anticus 


THE    PRINCIPLES    OF    MVODYNAMICS.  140 

major,  the  sterno-cleido-mastold,  and  the  acces- 
scry  hyoid  groups  of  muscles. 

(2.)  The  extensors  of  the  skull  are :  The 
rectus  capitis  posticus  minor,  the  rectus  capitis 
posticus  major,  the  superior  oblique,  the  trach- 
elo-mastoid,  the  splenius  capitis,  the  complexus, 
the  biventer  cervicis,  and  the  trapezius. 

(3.)  The  rigJit  rotators  of  the  skull  are:  The 
left  sterno-cleido-mastoid,  the  left  trapezius,  the 
right  splenius  capitis,  the  right  trachelo-mastoid, 
the  right  rectus  capitis  posticus  major,  and  the 
right  rectus  capitis  anticus  major. 

(4.)  The  left  rotators  of  the  skull  are  :  The 
right  sterno-cleido-mastoid,  the  right  trapezius, 
the  left  splenius  capitis,  the  left  trachelo-mas- 
toid, the  left  rectus  capitis  posticus  major,  and 
the  left  rectus  capitis  anticus  major. 

(5.)  The  right  adductors  of  the  skull  are  :  The 
right  rectus  capitis  lateralis,  the  right  superior 
oblique,  the  right  trapezius,  the  right  splenius 
capitis,  the  right  trachelo-mastoid,  and  the  right 
sterno  cleido-mastoid. 

(6.)  The  left  adductors  of  the  skull  are  :     The 


146  THE    PRINCIPLES    OF    MYODYNAMICS. 

left  rectus  capitis  lateralis,  the  left  superior  ob- 
lique, the  left  trapezius,  the  left  splenius  capitis, 
the  left  trachelo-mastoid,  and  the  left  sterno- 
cleido-mastoid. 

(b.)  (i.)  The  flexion  and  extension  of  the  skull 
take  place  mostly  at  the  cranio-atloid  joint, 
(ii.)  The  rotation  of  the  skull  takes  place  mostly 
at  the  atlo-axoid  joint,  (iii.)  The  adduction  of 
the  skull  takes  place  mostly  in  the  cervical  spine 
below  the  axis  and  In  the  cranio-atloid  joint. 

(c.)  The  term  adduction  designates  the  fact 
that  the  head,  as  It  is  bent  to  the  rlorht  or  to  the 
left,  approaches  the  body  on  the  side  to  which 
the  movement  is  miade.  The  term  ab-duction 
would  designate  the  fact  that  the  head,  as  It  Is 
bent  to  the  right  or  to  the  left,  moves  as  a 
whole  from  the  long  axis  of  the  body.  Either 
term  would  be  appropriate,  though  the  former 
has  been  used. 

137.  The  skull  Is  a  lever  of  the  first  order,  in 
which  :  (i.)  The  paiuer  is  derived  from  one  or 
more  muscles — assisted  at  times  by  the  weight 
of  some  part  of  the  head  ;  and  in  which  :      (2.) 


THE    PRINCIPLES    OF    MYODYNAMICS.  1 47 

The  weight  is  also  derived  from  one  or  more 
muscles — assisted  at  times  by  the  weight  of 
some  part  of  the  head.  For  instance,  when  the 
head  is  flexed,  the  weight  of  the  head  will  co- 
operate with  the  flexors  in  antagonizing  the 
power  of  the  extensors.  (3.)  In  fact,  any  one 
group  of  muscles  acting  on  the  skull  may  be 
considered  the  power,  while  the  antagonistic 
group  may  be  considered  the  weight. 

138.  When  the  body  and  the  head  are  in  the 
upright  position,  all  the  motor  muscles  of  the 
skull,  above  grouped,  co-operate.  The  antago- 
nistic groups  of  muscles  are  in  equilibrium. 
The  moving  components  of  the  groups  of  muscles 
antagonizing  are  equal. 

(i.)  The  moving  components  of  the  right 
rotators  equal  the  moving  components  of  the 
left  rotators.  (2.)  The  moving  components  of 
the  left  adductors  equal  the  moving  components 
of  the  right  adductors.  (3.)  The  moving  com- 
ponents of  the  flexors  equal  the  moving  com- 
ponents of  the  extensors. 

Examples  :      (i.)  The  trachelo-mastoids  will 


148  THE    PRINCIPLES    OF    MYODYNAMICS. 

antagonize  the  sterno-cleido-mastoids.  (2.) 
The  rio^ht  sterno-cleido-mastoid  and  the  rigrht 
trachelo-mastoid  will  antagonize  the  left  sterno- 
cleido-mastoid  and  the  left  trachelo-mastoid. 
(3.)  The  rotating  components  of  the  right 
sterno-cleido-mastoid  and  the  left  trachelo-mas- 
toid antagonize  the  rotating  components  of  the 
left  sterno-cleido-mastoid  and  the  riorht  trachelo- 
mastoid. 

139.  (i.)  The  j^etentive  components  of  the  mo- 
tors of  the  skull  zuill  co-operate,  as  zuell  as  the 
moving  components.  (2.)  The  displacing  com- 
ponents of  the  antagonizing  motors  of  the  skull 
will  also  a7itagonize  each  other;  and,  as  the 
antagonizing  groups  have  displacing  compon- 
ents acting  in  different  directions — that  is,  in 
opposite  directions,  these  groups  of  displacing 
components  will  counteract  each  other,  and  the 
cranio-vertebral  joint  will  be  in  a  condition  of 
permanent  stability.  (3.)  And,  in  any  case,  the 
retentive  components  are  greater  than  the  dis- 
placing components  of  the  head-motors. 


THE    PRINCIPLES    OF    MYODYNAMICS.  1 49 


THE    MVOMETER. 

136.  It  is  now  desirable  to  add  some  points 
of  experimental  evidence  afforded  by  a  machine 
that,  for  the  sake  of  convenience,  may  be  called 
a  myometer.  This  machine  is  made  as  follows  : 
(i.)  A  bar  of  wood  five  and  one-half  feet  long, 
whose  cross  section  is  about  one  and  one-half 
inch  square,  is  morticed  from  side  to  side  in  two 
directions,  for  about  five  inches  in  the  center,  so 
as  to  leave  a  small  quadrangular  piece  at  each 
corner — the  mortices,  or  slots,  meetine  at  rio;ht 
angles.  About  an  inch  beyond  the  mortices  on 
one  end  the  bar  is  cut  transversely  by  a  saw,  as 
far  as  the  opposite  side  of  the  mortice,  and  then 
slit  from  the  end  down  to  the  transverse  saw 
cut.  1  hen  an  opening  is  made  down  through 
to  the  mortices  as  large  as  the  central  opening 
of  the  two  mortices.  (2.)  Another  bar  of  wood 
about  two  feet  lonor  is  made  to  fit  into  one  of 
the  mortices  in  the  long  bar  ;  and  an  iron  pin 
goes  through  the  short  bar  transversely,  and 
plays  up  and  down  in   the  other  mortice.     (3.) 


l5o  THE    PRINCIPLES    OF    MYODYNAMICS. 

In  the  long  bar  are  fastened,  on  opposite 
sides  :  Two  screw-eyes  eighteen  inches  from 
the  center  of  the  mortices  ;  two  screw-eyes  nine 
inches  from  the  center  of  the  mortices  ;  two 
screw-eyes  three  inches  from  the  center  of  the 
mortices.  (4.)  In  the  short  bar  are  fastened, 
on  opposite  sides  :  Two  screw-eyes  eighteen 
inches  from  the  pivotal  pin  ;  two  screw-eyes 
nine  inches  from  the  pivotal  pin  ;  and  two 
screw -eyes  three  inches  from  the  pivotal  pin  on 
either  side.  (5.)  A  screw-eye  is  fastened  into 
the  thin  part  of  the  long-bar  directly  in  a  line 
with  the  long  axis  of  the  two  transverse  mort- 
ices. (6.)  The  long-bar  will  represent  a  fixed 
bone,  and  the  short-bar  will  represent  a  mova- 
ble bone  :  The  short  bar  can  be  used  as  a  lever 
of  anv  order. 

137.  (i.)  Select  three  spring-scales,  which 
have  equal  resistances  under  equal  tensions. 
To  determine  this  point,  hook  the  spring-scales 
together  and  make  tension  ;  if  the  scales  have 
the  same  resistance,  the  indicators  will  always 
point  to  figures  of  equal  value.  The  importance 
of  this  comparison  is  obvious. 


THE    PRINXIPLES    OF    MYODYNAMICS.  l5l 

(2.)  The  spring- scales  are  dynamometers  : — 
One  is  used  to  show  the  pressure  of  the  joint 
surfaces,  one  is  used  to  show  the  force  of  the 
contracting  muscle,  and  one  is  used  to  show  the 
resistance  to  be  overcome  by  the  movable  bone. 
That  is,  the  dynamometers  represent  the  power, 
the  weight  and  the  fulcrum.  (3.)  Also  select 
two  weights — one  weighing  one-half  pound, 
the  other  two  pounds.  When  the  two  bars,  the 
screw-eyes,  and  the  three  spring-scales  are 
properly  put  together,  they  constitute  a  7ny- 
onieter, 

138.  The  first  thing  to  do  experimentally  is 
to  resolve  the  force  of  the  dynamometer  repre- 
senting a  muscle  into  its  rectangular  compon- 
ents. This  may  be  done  by  the  myometer.  See 
Fig.  28.  Let  S  b  be  the  short  bar,  and  L  B  be 
the  long  bar  of  the  myometer.  Let  the  dynam- 
ometer P  represent  a  contracting  muscle,  and 
the  dynamometer  W  represent  the  resisting 
weight.  The  points  of  insertion  of  the  screw- 
eyes  may  be  seen  by  an  examination  of  the 
figure.     Some  experiments  may  now  be  made  : 

First.  Let  S  c  equal  L  c — that  is,  let  the  two 


1 52  THE    PRINCIPLES    OF    MYODYNAMICS. 


THE    PRINCIPLES    OF    MYODYNAMICS.  1 53 

osseous  sides  of  the  myodynamic  triangle  be 
equal.  And  let  the  myodynamic  angle  equal 
forty-five  degrees,  when  the  weight  puts  the 
contracting  muscle  under  strain.  The  dynam- 
ometer P  is  fastened  to  the  screw-eyes  at  L  and 
S,  and  the  dynamometer  W  is  fastened  to  the 
opposite  screw -eye  at  S.  Pull  on  the  dyna- 
mometer W  till  the  angle  S  C  L  is  a  right  angle, 
and  the  indicator  of  the  dynamometer  P  points 
to  the  figure  17,  then  will  the  indicator  of  the 
dynamoniete7'  W  point  to  the  figures  12.  Each 
of  the  rectangular  components  will  therefore  be 
twelve  pounds.  Hence,  P  will  have  a  moving 
component  of  twelve  pounds,  acting  in  the 
direction  W  S.  According  to  theory,  the  sum 
of  the  squares  of  the  tw^o  rectangular  compon- 
ents equals  the  square  of  the  resultant.     Hence, 

12^  -\-    \2^  T=    17% 

which  is  very  nearly  correct,  the  result  being 

288  ^  289. 
And  this  is  near   enough   for  theory  to  agree 
with  experiment. 

Second.  Let  a  c  equal  one  half  L  C,  and  let 
the  angle  ac  L  equal   a  right  angle,  when  the 


1 54  THE    PRINCIPLES    OF    MVODYNAMICS. 

weight  is  applied  to  d.  The  dynamometer  P  is 
fastened  to  the  screw-eyes  at  L  and  a,  and  the 
dynamometer  W  is  fastened  to  the  opposite 
screw-eye  d.  Pull  on  the  dynamometer  W  till 
the  angrle  a  c  L  is  a  nVht  anorle,  and  the  indica- 
tor  of  the  dynamometer  P  points  to  the  figure 
14,  then  ivill  the  indicator  of  the  dyniamometer  W 
point  to  \2\  nearly.  So  that  the  other  rectangu- 
lar component  will  be  6 J  pounds  nearly.  And 
P  will  have  a  moving  component  of  \2\  pounds, 

acting  in  the  direction  da.      Theoretically  \2\-\- 

2         .  22 

6i  =  14'.     But  14^  ^  196  ;    and    12^  -[-  61  = 

195^^.       Theory    and    experiment    very    nearly 
aeree  in  this  instance. 

TJiird.  Let  e  c  equal  one-sixth  L  c,  and  let 
the  angle  e  c  L  equal  a  right  angle  when  the 
weight  is  applied  to  f.  The  dynamometer  P  is 
fastened  to  the  screw-eyes  at  L  and  e,  and  the 
dynamometer  W  is  fastened  to  the  opposite 
screw-eye  f.  Pull  on  the  dynamometer  W  till 
the  angle  e  c  L  is  a  right  angle,  and  the  indica- 
tor of  the  dynamometer  P  points  to  the  figure 


THE    PRINCIPLES    OF    MYODYXAMICS.  100 

1 8 J,  then  will  the  indicatoi^  of  the  dynamometer 
W point  to  i8  nearly.  So  that  the  other  rect- 
angular component  will  be  3  pounds.  And  P 
will  have  a  moving  component  of  18  pounds, 
acting  in  the  direction  f  e.      Theoretically  18^  -\- 


■^  n  -^ 


i8i.  But  i8i  =  333;,;  and  iS'  +  3^ 
Experiment  substantiates  theory  by 
agreeing  with  it  very  nearly. 

139.  (a.)  The  second  thing  to  do  experimentally 
is  to  apply  the  principle  of  the  lever  to  the  resolu- 
tion of  the  force  of  the  dynamometer,  representing 
the  contracting  mnscle,  into  its  rectangular  com- 
ponents :  The  principle  is  that  the  force  in  the 
continuity  of  the  lever  equals  the  sum  of  the 
forces  acting  at  the  ends  of  the  lever. 

(b.)  Let  S  b  (Fig.  29)  be  the  short-bar  of 
the  myometer  ;  let  L  B  be  the  long-bar  ;  let  P 
be  the  dynamometer  representing  the  muscle  ; 
let  F  be  the  dynamometer  representing  the 
pressure  that  one  bone  makes  on  the  other  ; 
and  let  \\'  be  the  weight.  L  B  represents  the 
fixed  bone,  and  S  b  represents  the  movable 
bone  :  Then  S  b  is  a  lever  of  the  first  order. 


1 56  THE    PRINCIPLES    OF    MYODYNAMICS. 


THE    PRINCIPLES    OF    MYODYNAMICS.  1 57 

140.  The  power  of  the  dynamometer  P — or 
the  force  of  the  contractinor  muscle — is  resolved 
into  a  moving  and  a  displacing  component.  The 
moving  component  is  a  retentive  component. 
The  displacing  C07np07ie7it  is  resisted  by  the  ar- 
ticular liganuiits,  which  put  the  stress  of  the 
displacing  conponent  on  the  joint-sitrfaces  i7i 
addition  to  the  stress  of  the  moving  cofuponejit. 
Hence,  the  resultant  of  the  movifig  and  displacing 
componejits — that  is,  the  force  of  the  contracting 
muscles — zuill  appear  as  stress  on  the  joint- 
snrfaces  : 

(i.)  For,  if  we  could  suppose  that  there  is  no 
myodynamic  angle,  plainly  the  force  of  the 
muscle  and  the  force  of  the  weicrht  would  be  in 
the  same  direction,  and  their  sum  would  consti- 
tute the  pressure  on  the  joint-surfaces. 

(2.)  Make  the  myodynamic  angle  acute,  and 
it  will  be  found,  by  the  myometer,  that  the 
weight,  plus  the  indicated  figures  on  the  dyna- 
mometer P  will  equal  the  indicated  figures  on 
the  dynamometer  F. 

(3.)  Make  the  myodynamic  angle  right,  and 
it   will   be  found,   by   the    myometer,   that   the 


1 58  THE    PRINCIPLES    OF    MVODYNAMICS. 

weight,  plus  the  indicated  figures  on  the  dyna- 
mometer P,  will  equal  the  indicated  figures  on 
the  dynamometer  F :  The  dynamic  angle  will 
be  acute,  and  the  weight  will  have  a  resisting 
and  a  displacing  component ;  the  displacing 
component  will  be  put  on  the  joint-surface  by 
the  articular  ligaments  ;  the  resultant  of  the 
resisting  and  displacing  components — that  is, 
the  force  of  the  weight — will  appear  as  stress  on 
the  joint-surfaces. 

(4.)  Make  the  myodynamic  angle  obtuse,  and 
it  will  be  found,  by  the  myometer,  that  the 
weight,  plus  the  indicated  figures  on  the  dyna- 
mometer P,  will  equal  the  indicated  figures  on 
the  dynamometer  F. 

141.  Since  the  principle  of  the  three  orders 
of  the  lever  is  the  same,  it  must  follow  that  the 
conclusions,  in  reo^ard  to  the  resultants  of  the 
components  of  the  power  and  weight  acting  on 
a  bony  lever  as  above  enunciated  in  regard  to 
the  lever  of  the  first  order,  are  also  true  rela- 
tively in  regard  to  the  resultants  of  the  com- 
ponents of  the  power  and  weight  acting  on  a 
bony  lever  of  the  second  or  third  order — that  is, 


THE    PRINXIPLES    OF    MYODYXAMICS.  i:)9 

the  sum  of  the  forces  at  the  ends  equal  the  force 
in  the  continuity  of  the  lever. 

142.  In  order  to  determine  the  resistance  of 
cancellous  bone,  I  have  cut  off  the  ends  of  long 
bones  and  subjected  them  to  pressure.  A  few 
points  in  regard  to  these  experiments  are  suf- 
ficient for  our  present  purpose.  The  results  are 
only  approximate.  The  bone  ends,  in  one  case, 
will  have  more  power  of  resistance  than  the 
bone-ends  in  another  case.  I  make  the  follow- 
ing record  in  regard  to  the  ends  of  a  well-formed 
and  apparently  healthy  femur  : — 

(i.)  The  head  was  cut  off  at  its  junction  with 
the  femoral  neck,  making  two-thirds  of  a  spher- 
oid, whose  diameter  was  about  one  inch  and  a 
half.  It  contained  about  1.7  cubic  inches  of 
cancellous  bone.  The  cut  surface  of  the  head 
was  put  on  a  resisting  surface  and  pressure 
gradually  applied  to  the  top  of  the  head  by 
means  of  a  steel  lever.  When  the  pressure  was 
about  400  pounds,  the  top  of  the  femoral  head 
began  to  flatten,  and  continued  to  be  depressed 
as  more  and  more  force  was  applied  till  pressure 
was  about  2,200  pounds,  when  the  femoral  head 


l6o  THE    PRINCIPLES    OF    MYODVXAMICS. 

had  settled  down  one-half  inch,  had  expanded 
on  all  sides  to  a  diameter  of  an  inch  and  three- 
fourths,  and  had  about  ont^-tliird/^r//'^//)/ broken 
off: — Let  me  call  attention  to  the  fact  that  this 
amount  of  pressure  (2,200  pounds)  is  only 
a  little  greater  than  the  amount  of  pressure 
(1,700  pounds)  derived  previously  from  a  theo- 
retical basis. 

(2.)  The  neck  and  the  greater  part  of  the 
trochanteric  region  of  the  same  lemur  were  cut 
off,  and  subjected  to  pressure  in  the  same 
manner  as  the  head  : — The  base  of  the  piece  of 
bone  was  two  and  one-fourth  inches  by  one 
inch  and  a  half:  The  summit  of  the  neck  was 
about  one  inch  in  diameter.  When  the  pressure 
was  about  600  pounds  the  summit  of  the  neck 
began  to  yield,  and  when  the  pressure  was  over 
2,200  pounds,  the  summit  of  the  neck  had 
settled  clown  about  three-eighths  of  an  inch, 
leaving  the  trochanteric  region  quite  unchanged. 

(3.)  The  condyles  of  the  same  femur  were  cut 
off  transversely  to  the  shaft  of  the  bone  : — The 
cut  surface  of  the  piece  was  about  three  inches 
by  two  inches  :   the  depth  of  the  internal  condyle 


THE    PRINCIPLES    OF    MYODYNAMICS.  l6l 

was  one  inch  and  a  half,  while  the  depth  of  the 
external  condyle  was  one  inch :  This  permitted 
pressure  to  be  made  only  on  the  internal  con- 
dyle. When  the  pressure  was  some  600  pounds 
the  internal  condyle  gave  way  and  settled  to  a 
level  with  the  external  condyle  :  And  the  two 
condyles  then  sustained  a  pressure  of  over 
2,200  pounds.  As  the  cut  surface  was  not  quite 
transverse  antero-posteriorly,  the  condyles  split 
nearly  in  two  from  side  to  side.  It  would 
appear,  therefore,  in  this  case,  that  one  condyle 
would  be  inadequate  to  sustain  the  pressure  of 
the  quadriceps  extensor,  as  obtained  above,  by 
theoretical  considerations — namely,  over  1,000 
pounds — and  that  both  condyles  of  the  femur 
would  be  adequate  to  do  the  work  required  by 
the  myodynamic  relations. 

143.  The  practical  conclusions  are  :  (i.)  That, 
under  ordinary  conditions,  the  cancellous  tissue 
of  bone-ends  can  sustain  the  pressure  applied  to 
it  by  the  daily  use  of  the  muscles.  (2.)  That 
the  structural  conditions  of  the  bone-ends  may 
so  change,  on  account  of  disease  or  injury  that 
the   action  of   the  muscles   may   cause   absorp- 


1 62  THE    PRI^XIPLES    OF    MYODYXAMICS. 

tion  and  deformity,  (3.)  That,  when  sufficient 
external  violence  is  added  to  the  force  of 
contracting  muscles,  sound  cancellous  tissue 
may  be  more  or  less  broken.  (4.)  That  this 
broken  cancellous  tissue  may  be  more  or  less 
impacted  by  having  the  plates  and  arches  of 
bone  interpenetrate  and.  overlap. 

144.  Finally,  attention  may  be  drawn  to  the 
important  bearing  of  myodynamics  to  fractures, 
dislocations,  and  orthopoedic  surgery.  A  com- 
plete understanding  of  these  three  great  de- 
partments of  surgery  can  not  be  had  without  a 
knowledofe  of  the  Prinxiples  of  Myodynamics. 


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